I am very interested in the outcome of your search for the source of the engine vibration. I too have had a engine vibration that fits your description. I have had this vibration in my 95 S500 since it was around 45,000 miles, and the car currently has 62,000 miles. I have brought the car to the dealer on many occassion without success. These are what was replaced: Engine mounts, plugs and rotor, reset computer fuel injection pressure checked. I hope that you're able to find the problem.
By the way does the vibration occur only in park and nuetral? Because, I can only feel the vibration in park and nuetral in my car.
Good Luck.