You should check the motor and/or tansmission mounts. You can do this by opening the hood and watching the motor while you blip the gas in drive. The engine should only move slightly and not abruptly. If it moves more than an ince or so and moves as though nothing is damping the motion, then the motor mounts and transmission mounts must be replaced at the same time. If you don't the newer mounts will get detroyed rapidly.