Thought I'd chime in with my views.
Having seen your car, perhaps I can describe the symptoms further for the others :
With A/C off, no significant vibrations, if any, they are very subtle.Regardless whether in gear or not.
With A/C on, in PARK, slight vibration felt from within the car.
In gear, the vibrations are more pronounced.
I had the opportunity of looking at the engine bay with Andy in the driver's seat.
With A/C on, and the car in gear, the whole engine vibrates severely, such that the airfilter housing/inlet is bouncing all over the place.
I diagnosed shot engine mounts and suggested a 2nd opinion from a reliable Merc indie.
It seems that with the compressor on, the engine is loaded and as such the vibrations are more pronounced.
Anyone care to chime in with their thoughts?