A little bit of vibration in drive is normal. The engine is under load in a low RPM situation. When in neutral, the vibration goes away because you are unloading the engine. If the vibration is significant, you may want to investigate the motor mounts.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record