2000 w208 CLK55. I have replaced engine and gearbox mounts and had the wheels balanced. I've also replaced the MAF recently and changed the oil. I'm suspecting it may be the propshaft, but I can't find any info online with people reporting the same symptoms.

So I'm getting a constant vibration, no matter what speed or rpm I'm at. It's been doing it intermittently for the past few months, but suddenly got worse on Friday.
It seems to get worse with engine speed and load. There's a slight misfire at idle, but I don't believe it's causing this? The vibration is there when I'm at cruise, and gets worse when accelerating, and even when I'm hard on the brakes. It's not coming from the brakes, as I can't feel it through the pedal.I've knocked it into neutral whilst I'm driving and the problem remains. I've tried shifting through the gears manually and it remains.

I can hear a faint ticking coming from under the car when I'm at very slow speeds. It goes away or is drowned out by road noise as speed increases.

There's a bit of drivetrain jerk if I let off the throttle and get back on. When doing this, there's also a delay between pedal action and engine/drivetrain response.

I also have an issue when it comes to slowing to a stop. As I come to a stop, I'm pressing the brake very lightly. The car feels as though it's fighting the brake, and yo-yos around, speeding up and then slowing down, until I finally come to a stop. It doesn't come to a stop smoothly either. Once stopped, it's fine.

Sometimes I get a judder when moving off after shifting from park to drive. It feels as though the brakes are held on. It requires additional throttle to overcome the stiction, and then it's fine.

Doing a three point turn can also be a bit slow. The drivetrain takes a second or so to catch up. Trying to perform a turn like this quickly can lead to wheelspin in forward and reverse, so the only option I have is to do it slowly.

The symptoms make it sound as though the propshaft couplings are shot. I've been under the car recently, and the propshaft couplings looked ok. I will be changing them regardless.

 

Things just got interesting... I had it up on axle stands today and put it in drive. There was a distinct ticking from the rear end. I applied the hand brake and gave it some revs. The ticking turned into serious noise and vibration. The propshaft was spinning, but the driveshafts were not rotating smoothly.
I reckon the diff's running dry. No obvious signs of leakage though. See vid:

https://www.flickr.com/photos/157348472@N05/51400796133/
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Pulled the diff out and it appears to be fine. No broken teeth, no sign of wear, just 20 year old oil.
There's a small amount of backlash, but I'm not sure it's bad enough to be causing my issue?

 

Propshaft flex coupling(s) and center support bearing if present.

 

I believe that video shows your ABS/Traction control is functioning correctly.

 

They're fine.

Nope. Traction was turned off. Calipers were removed to eliminate any such issue. You can hear a distinct clacking noise. If you think that's normal then I have news for you...

I think I may have found the problem. The left inner CV was sticking in certain positions. I lifted the boots to find a lot of runny grease. I packed them with new grease and left it at that. I can't find new CV joints, and I don't fancy paying for new complete shafts.
Whatever I did seems to have solved the problem. I've just done a 300 mile trip and there was no vibration. Let's see how long it lasts...

 

@Hooblah what is the root cause and what have you done to address it?

 

No idea. 20 years and 118k probably.