Sounds like a misfire. Fouled plug?? Give it a few "Italian tuneups" & see if there's a difference. Look under the hood when it's really dark at night & see if you've got some stray sparks when there's a load on the engine. Look especially at the distributor cap & coil wire.

Water in the gas?? Try a little drygas.

When the car's in neutral, there is very little side load on the engine & trans & diff mounts. In drive, they'll be torqued to one side. MB engines run clockwise (viewed from the front) and the torque reaction would be in the opposite direction. Possible a deteriorated mount? Get under there & take a look.

Keep us posted,

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Norm in NJ
Next oil change at 230,000miles