Thanks for your comment about the Michelin XGT's. My mechanic says they are like Goodyears - why would anybody put them on a Mercedes???
I have a similar type problem on my 1987 300D (247Kmi). The front and rear suspension have been nearly 100% rebulit. There's no rubber in the front that hasn't been changed, the only thing in the rear original is the subframe bushings and inner control arm bushings and top diff. mounts. Shocks were changed about 60Kmi with HD Bilsteins. Rears were improperly mounts and now one leaks very small amount. Front wheels bearings replaced, rear are original. Everything seems tight. Dealer shop foreman on a previous issue identified my transmission as a problem. He didn't like the way it shifted. My mechanic says there is something wrong with it too, and there's a klunk when rotating the driveshaft that neither say is the driveshaft - I have another post on this in the Tech section asking if output shaft play can cause vibrations? My vibration is in the steering and gas pedal. Changing into neutral or 3rd while coasting or going downhill (when its most noticable) does not change the problem.
If its a battle you plan to win, don't lose hope. But don't put your life savings or complete emotions on it either. Ever hear of the story of the new car that had a vibration (so under warranty). First things wheel bearings, etc replaced. Then driveshaft. Then trans. Then engine. Then problem was gone, owner so wore out he traded it in for another new car.