Any ideas?? Once car is warm, there will be an odd shutter (felt in the seat and floor pan) first when the car shifts from say 1st to 2nd, or 2nd to 3rd gear (low speeds). This seems to turn into a pretty strong vibration when driving at highway speeds (very annoying). Thanks for reading!

 

I'd like to know too. Mine started doing it too, at the lower speeds shifting from 1-2, and 2-3. Haven't noticed anything at highway speeds. Gonna have the transmission checked maybe its slipping. Wonder if it could be a bad motormount?

 

tranny is slipping, soon to fail, read the tranny failure thread. mine shuttered for 30k miles before i got it replaced.