I had the same issue with my 94 C280 and chased this issue for over a year. My car had factory AMG 170 rims and would vibrate between 55-60mph. New control arms, bushings, roadforce balance, etc....nothing would help. The culprit ended up being a crack in the rear control arm mount where the spot welds are located. I had the crack welded up and after tat the car was smooth as glass at all speeds. Good luck.