I had a similar problem with my 87 300 sdl. I had it to 2 shops, one an independent that thought it was the driveshaft, and the dealer, who....thought is was everything and started replacing things...to no avail.

My problem turned out to be a bad left side half shaft. You could grab it and move it back and forth.

Drive trains are difficult to troubleshoot. It could be the shaft itself out of balance (if someone worked on the shaft before), the center carrier bearing, the flex disks, half shafts, and even the rearend.

Make sure you don't have a wheel out of balance. Drive train problems are usually found when under power, so if you coast and have it, it probably a wheel.

My problem would only occur at 75MPH plus. That makes it difficult to troubleshoot for a dealer in the suburbs. It's rather dangerous to put it on the rack and run the drive train up to speed.

After all that worthless chat, I would try to pull and tug on each half shaft and see if you notice any different between them.

If one is loose, I would order one from the MB salvage yards on the net...used ones are a lot cheaper, and "swap it out".

Joe