Repair Failure
 

2 new Lemforder Flex-discs, new OEM center support and OEM (fag) bearing
and the vibration during acceleration between 35-50 is still there.
I scribed the drive shaft before dis-assembly and put it back exactly where it was.
To be entirely forthcoming I think the vibration is a little worse.
U-joints were fine and nothing else looked wrong. I replaced the front disc last month, could it go bad in 1 month? It looked new (cause it is) and showed no play.
Any thoughts?
The PO told me he had the front disc replaced but that was 6 years ago and there has been no vibration like I have had for the last 3 months.

Let's get back to basics and establish some data.
Do these vibration's frequency depend on engine rpm? Speed of vehicle?
How are the motor mounts, trans mount, differential mount?
Are there any loose parts under the vehicle? Exhaust from front to back... exhaust hangers?
Does the vibration appear in any one gear? 3rd... 4th?

As Above
 

Also if the above does not pin point to the problem, take it to a tire shop that spin balances with the wheel on the car, this gives one the ability to look at each wheel assy. as a whole, seperate from the rest or the car.

I'm going through this same issue with my car. I've replaced Flex discs, center support and bearing, motor mounts. All MB factory parts. Next up is trans mount, rear diff mount, centering bushings. If that doesnt fix it...then I'm going to change the universal joint. Its not the tires because the vibration only occurs on acceleration. It is usually ANY acceleration even the slightest push on the accelerator causes it. I can be cruising along at 30-60 mph and its smooth as silk....step on the peddle slightly and a vibration that you can feel through the seat happens. Back off slightly to cruise again and its gone. The harder you accelerate....the more you can feel the vibration. Its a frustrating problem to narrow down. I will have rebuilt the whole driveline by the time I figure it out :(
John

Put enough weight in the car to make the drive axles run straight and see what happens. If the vibration is diminished or goes away, it's time for axles. A driveshaft vibration is typically speed related, not load related. The axle ratio in combination with the three-lobe CV joints will give a frequency very close to the DS rotation speed.

Thanks for the info. Now I know what's wrong with my car!
You have been driving it.
Seriously, I'm having the exact same problem, as you have said.
It's only happens during acceleration I can coast at 42mph and it's smooth as silk, the instant I touch the pedal there is the vibration.
The slightest incline in the road between 35 to 50 and there is the vibration.
I can cruise at 42 while it's vibrating and slip it into Neutral and then NO vibration. Tires are almost new and balanced. I did replace the axle shafts 6 months ago and after there was no vibration. I used some really cheap FEQ brand and had bad luck with them but the 3rd pair have been fine so far. I am going to re-boot the originals, switch them from right to left and get the FEQ outa there (pun intended). I haven't done the engine mounts in 2 years and the trans mount looks fine as do the diff. mounts, new muffler and other hangers. I am almost out of ideas.
Good luck with yours and please let me know if you find out what the fix is.

Im had a problem one time with my 76 300D. I replaced the tranny mount with new Meyle mount that i had just because i figured it should be changed as the mount appeared original.

The Meyle tranny mount gave me all kinda of driveline vibrations. Needless to say i installed the old original mount and the vibrations went away.

I put that same Meyle mount in my 83 300D when i put the motor in and i have a bad drive-line vibration from about 50-70, after 70 it kinda fades away. Im gonna buy a mount from mercedes to see if it gets rid of this vibration.

Be sure to use the correct Mercedes trans mount, I did not have good experience with Meyle.