I had the devil of a time finding a "harmonic" vibration on my '85 300cd. Did everything I could think of and got no help on diagnosis from indepentent shops as well as mercedes dealer. Finally replaced two rear half shafts with rebuilt units - that did the trick. My car had about 138k miles at the time and apparently, they were just tired. Can be bought for about $200 each at Fastlane. But....consider replacing driveshaft center bearing first. Also look at rebuilding main universal joint and/or have driveshaft balanced (about $200 for those things).
Good luck with your mission and look at half-shafts if all else fails.