I have had the problem at about 40 mph. I started expermenting and on my car (the 84 500SEL) raising the rear of the tramsmission about 3/16" made it almost go away. You can also try lowering the center support by using washers, and changing the angle of the diff using washers. The center support and diff tricks didn't help mine at all, but it didn't hurt it any either, so they both went back to original. There is a procedure to allign the drive line but it requires special tools which I have never been able to find anywhere. Even the local dealer said they didn't have them. They just said to replace all mounts from front to rear and install a new driveshaft and that would fix the problem. I didn't wait for them to give me an estimate.

Paul

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84 500 SEL (307,xxx miles)