Honestly, I think it might be normal. Auto's shake when in gear but not moving. When the car is not moving it drags on the engine, slowing the rpm's down enough that you can feel each piston firing, causing the engine to vibrate more violently.

It does this mainly when the car is warmed up (the idle drops as the car warms up) and when the AC is on (the ac pump drags on the engine, also slowing the rpms down).