In hopes of finding this problem for my self and others I will give some more clues on my 84 500SEL. Remember, my 91 300D also does the same at the same speed, and under the same conditions.
I only feel the vibration at about 20 MPH. It comes and goes very quickley. It only happens at moderate to heavy acceleration. I don't feel it a light acceleration or WOT. At WOT, it passes 20 MPH so quickly that I really can't tell if it happens or not.
Here are other facts to help.
Following have been replaced
1. Used transmission from an 98 420SEL.
2. New flex disk front and rear.
3. New center support and bearing
3A. Rebuilt and balanced drive shaft.
4. Rebuilt axle shafts (two times)
5. New tires and different wheels.
6. New rear axle bearings.
7. New engine, trans and differential mounts.
8. Complete rebuilt front end.
9. New rotors and brakes all the way around.
10. And other things I can't think of right now.

The above things were not done to fix this vibration, just part of a rebuild on a car with 250K miles. The car runs great, drives great, is smooth on the road with no vibrations. We just got back from a trip to Flordia with was about 1500 miles round trip, and averaged 21 MPG go 75 to 80 the whole trip.

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84 500 SEL (307,xxx miles)