My 91 300E had a bad vibration too. Waiting at stop lights I thought I might be shaked right out the window... A couple hours combing throught the archives lead me to zero in on the motor mounts. I order them from FastLane and replaced them myself - a fairly simple procedure which has been covered in many threads. Replacing the motor mounts was the best bang for the buck repair I've done so far on my car. The diffrence with new mounts was amazing, so much so I had to drive around for the next couple days with the stero off to make sure the engine was in fact running... smoooth.
Cheers for now