New transmission????
 

I have 2014 E350 4 matic sport, 24940 miles...Car is running absolutely great, no issues until a few days back...With even sligh turn left or right i felt strong vibration on steering, visited dealer and they replaced planetary gear/transfer case/... Picked up car and after 10 miles,same problem even worse...Now dealer open case with MB and looking i am gonna need new transmission... Waiting for dealer call...My warranty expires 8/31/19,and problem like this makes me nervous... I asked master mechanic if replacing parts are improved and he said NO...Any advice??So much for Best or nothing... I was told every 4matic has the same issues,even S class...I love this car but????Are the newer models having the same issues...

I had the same issue, but after leaving the car for few days not driving, soaked the new part and the issue hadn't came back again.

Well, you're already heading down this path with the dealer but I'm not sure how the transmission falls into this. Seems that it would have troubles regardless of whether you're turning or not. As I remember power from transmission transmits thru a transfer case to the front wheel halfshafts, and out the back by driveshaft to the rear differential. The transmission doesn't know if the car is going straight or turning.
I guess front hubs and steering rack attachment have been checked by now but if theres vibration at turns, then the question is what is seeing side loading that is overwhelming that part or parts? All suspension is ok?

All valid points...Dealer drove the car and made diagnostic,and they checked everything... I am confident in their ability to diagnose problem...I am just suprised this is all happening with only 25000 miles...

Things happen, regardless of mileage unfortunately.
At least you've got it under warranty. Good luck with it, I hope you post if transmission gets replaced.

Thanks,i will post progress... Reading more about this issue,i am surprised how many 4 matic models/from 2006/, regardless being C,E or S class is involved...And inability from company like Mercedes Benz to fix it...

My guess is the problem is with the center differential lock disk jerking as it is not slipping smoothly. Vibration appears at turns as this is when you generate speed difference between front and rear axles and the lock has to slip to allow the speed difference between the axles.

A simple test could reveal this. If you let tire pressure way down on front or rear tires you would generate speed difference between axles that could show the vibration also when driving straight.

My guess is the problem is with center differential lock jerking. It appears at turns as this generates axle speed difference between front and rear.

A simple trial wiould be to let tire pressures way down on one axle tires to generate speed difference and perhaps wibration then appears driving dead straight too.

Update
 

Just got call from dealer...MB wants now transmission fluid sample and EEprom data from transmission...