Hello all, Just wanted pass on the guidance I was able to gather that helped me replace the front u-joint propshaft driveshaft in my 2012 C350 4matic Coupe (which should be the same for a W204 Sedan and maybe several other 4matic's??). I scoured the internet for help and pieced together a few replies from several different threads on this site and a couple other sites. Thought it might help someone to get it all in one post. *This might not be in perfect order but all needs done, also I'm not a certified mechanic just a home garage parts changer so the terminology used might not be perfect, hope this helps:
**SAFETY IS PRIORITY #1 so inspect your jack stands/lift etc.. and put them in a SAFE/APPROVED location under car! I used attachment point of lower control arm and frame which might not be wide enough if not on smooth level ground. RE-INSPECT them REGULARLY to make sure they have not settled or shifted. While in middle of this project and searching for help online at 2am I saw a headline on a local news site that read "Man killed, crushed under car in parking lot". So please be SMART when doing something like this! Also recommend: keeping pets away from work area, had two cats get in a scuffle right beside one of my jackstands one night...luckily it was only a warning sign of what not to do! Also recommend: setting up a back-up/secondary support(heavy duty wood blocks, concrete blocks, etc..) slightly lower than jackstands/lift just in case your cats are as careless as mine.. seriously though DONT attempt anything you're knowledge, skill, or equipment aren't capable of!
You'll need to raise and support vehicle SECURELY with enough room for you to move around comfortably underneath.
1. Then remove all plastic splash guards from the front wheels back.
2. Then you'll want to spray ALL nuts, bolts, screws, anything with threads on it that is listed below with some type of penetrating catalyst(Kroil, PB Blaster, etc) at LEAST once, and give it 10-15min before attempting to remove!! *Highly recommend being slow and deliberate with this whole process so you don't damage anything and because there's a 3600lb car over your head!!
3. Next remove both exhaust clamps upstream of muffler, had to cut mine off, *save the barrel shaped nut for re-attaching clamps with new bolts
4. Next remove passenger side exhaust pipe at the manifold collector flange and the exhaust mount bracket. *I used a wire wheel attachment on a drill on seam to help free exhaust from manifold, but it did make a mess.
5. *Carefully remove both 02 sensors and hang somewhere up out of way, place exhaust pipe somewhere safe for re-install
6. Disconnect driver side exhaust bracket at the two higher bolts so that it will allow engine/trans to swing towards driver side
7. Remove passenger exhaust heat shield(3 bolts, 1 facing skyward on top of front diff).
8. Next Remove the three or four other small brackets positioned in close proximity to driveshaft.
9. Next disconnect the transmission wire that loops under driveshaft.
10. Then flat aluminum cross member(sub-frame 4 bolts)
11. Remove anyhting else between you and driveshaft i didn't mention

12. *Mark rear end of driveshaft on shaft and transfer case, and note exact orientation of front end attachment plate using stamped letters and its shape. also note flat sides of shaft mount plate in relation to the two (non threaded)holes on differential mount plate. *take pictures and hold new shaft up beside old one and mark how you going to install it.
13. *Depending on condition of your shaft, you can either carefully cut it out like I did then remove the 4 bolts and the mount plate, or(see below first) you may be able to remove the bolts, press it rearward towards transfer case, and maneuver head into temporarily vacated space where catalytic converter section of exhaust lives.
14. *Spray again and very carefully remove the four bolts attaching the shaft to the front diff one at a time. MB says bolts are one-time use and should be replaced. I was able get them out without damaging so decided to re-use them....IF you make this decision you're going to want to put each bolt back in the same hole it came out of. I traced head of new driveshaft and marked bolt holes/stamped letters onto cardboard then punched out bolt holes and stored bolts in cardboard replica for re-install. *you may need to go back and forth from Park to Neutral to remove bolts, in Neutral I carefully rotated shaft to access each mount plate bolthead by using a prybar wedged in between boltheads of the flexdisk, then back to Park to remove 1 bolt, then repeat.
15. Once old shaft is free and clear I recommend cleaning area, I used wire wheel above so I had a mess of rust and wire sheds to vac up/blow off
16. *take a few deep breathes....
17. Next center a capable jack under the cross tunnel transmission mount bracket and remove the 8 bolts.
18. Next you'll want to carefully remove the bolt on the top passenger side corner above the differential. You gonna need every mm of space
19. Then remove the rubber cap and boot from new shaft and put it on the transfer case output shaft
---Now I'm gonna tell you what has to happen.....then I'm gonna copy and paste my words from a message I sent the night I finally got it installed that explains how I did it and what tools I used.
----*Fitting the new shaft into this space is about precision parts alignment not brute force! The only way it fits is by getting the transimission/transfer case/rear of engine low and towards drivers side of tunnel so you can get the rear end of new shaft started onto the splined output shaft, then swinging the head end around manifold flange from the void space left by vacationing cat converter and sideways to a position on top of front diff case, then getting rear end a little further onto output shaft, then pulling the head/front end rearward and down to rest on the face of the top passenger side corner of differential housing in the space left by vacationing bolt, *At this point I had to carefully swing transfer case/transmission/rear of engine back to center of tunnel and raise transmission back up to original height, then had to tap rear end with rubber mallet so it fully seated onto output shaft while guiding head end down to a position where the two mounting plates were at same height (I had maybe 3mm between the two mounting plates), then pull the shaft towards the front of vehicle until the two plates touched and the shaft mounting plate was resting inside the dish or small lip/flange on the front diff mounting plate.
===To pull that off I had to have transmission as low as possible and as far towards the drivers side as possible without damaging the lines, linkage, rear driveshaft, flexdisk, etc...
20. *I then SLOWLY CAREFULLY lowered transmission all the way down until it basically hung from the rear driveshaft and the motor mounts. *You may want to try without going ALL the way down and ALL the way over first? It really accents the flex part of the flexdisk! I tried it for 6 days, dont recommend trying for that long
---My words from message: I took one of my 600lb clamps apart and put the pieces on backwards to make it a spreader and used it up at the tranny bell housing instead of back behind the mount plate where I had been using the prybar and wood blocks. Then with spreader and big prybar, and with transfer case/tranny/engine hanging by motor mounts and rear shaft, I carefully swung engine as far as I could towards driver side of tunnel. Used a medium prybar on the flexdisc boltheads to rotate the output shaft and get the flat spot on shaft mounting plate horizontal from 5-7 o'clock. The pins on my spanner wrench out of my grinder handle fit perfectly in the groove on the rearend of the shaft so I tapped the butt end of it a few times with a rubber mallet and saw it creeping further onto the output shaft. Then the tapered section where the shaft swells out wedged against the tranny housing....again.... so I put my straight edge back up against shaft....again.... used my one eyed/shooting range/eagle vision to see what direction things needed to move....again.... and for the first time in probably 15 attempts over the last week, I realized I had it! Carefully removed my spreader, swung everything back into center of tunnel and raised it all back up to flatten out the shaft the one or two degrees it needed to line up, and it slid right on.
21. Then I used prybar to rotate shaft to correct position matching my cardboard replica, so bolts would go back into same holes
22. Next using blue threadlock I re-installed the 4 bolts using a torque wrench tightening to MB's spec of 37lbft(40nm)
23. Put it all back together
24. Lastly:

Well that escalated quickly! Sorry so long but it's a difficult task, dealing with expensive parts and I'm not a very gifted explanative writer... This took just about every bit of patience this 42yr old had! but I only broke 6-8 things in between attempts and got it done. Thanks to all who contribute to this site. Hope this helps someone someday.. Sincerely, DMurph