Well - one idea could be the axel shafts? Doesn't this car have a split rear end? When they start to go bad (the CV boot) they can vibrate. Or if one gets bent a little. Does it vibrate, or hum? I had a bad wheel bearing on the front of my 80 300TD that hummed loudly. Other places to look could also be the flex discs on the drive shaft. I am not super familar with this late of a W124, so I could be way off base.

Good luck !