What would cause a 92 Geo Prizm to vibrate badly when in drive?

Hi, I recently bought a 1992 Geo Prizm and it vibrates pretty bad when in drive. It's fine when in park or reverse or when the car is in motion but as soon as it stops at a light it vibrates again. I have checked the motor mounts, cylinders and plugs and all are fine. I cant figure out what else it could be. It runs smooth and is quiet except when the gear is in drive. It seems to vibrate less when the engine has warmed up pretty well. It was well taken care of by previous owner. Any suggestions? Thank you.
-Michael