Acceleration/Vibration Noise '90 300SE

When I accelerate from a dead stop, I usually get a intermittent vibration noise that always disappears above 15-20 mph, and never occurs at higher speeds. If I accelerate from a stop very slowly (which for a 300SE is VERY VERY slowly), the noise will not appear as often.

The first time into the shop, the motor mounts were replaced. This solved a different (constant) vibration problem, but not the one above.

The repair shop thought that the torque movement of the engine might be causing contact between the exhaust system and the frame (or something like this). I did not investigate this--not sure how someone could...

Tires have been rotated and balanced--pretty sure they are not the problem.

If I had to guess (and I'm no mechanic) I'd guess that a driveshaft bearing or joint is loose...but this is just based on gut feel, and little knowledge..

Any ideas?