Felt vibration at speeds >= 45 mph
I have a 1984 300D TurboDiesel with recently replaced engine mounts and engine shocks. At approximately 43-45 mph and greater I feel a vibration that creates a beat frequency (low pitch) with the engine. The service shop said that they balanced the tires. After driving about 500 miles, I sense the vibration is associated with tires/wheels, but I am not sure. I can shift to neutral at highway speeds (down hills and such) with ~800 rpm engine idle. The vibration remains distinct. The lead Mercedes technician does not suspect a drive shaft imbalance. How should I proceed to troubleshoot and remedy this problem with the least cost?