Thanks for the reply.

An interesting thing to note is that the car, when the engine is completely cold, idles around 2000rpm while in park after being started. The RPMs don't flutter around so I don't think it is a vacuum problem. I assume that there must be a valve trying to regulate engine temp (which would cause high idle)... If this is true, then the fact that the RPMs are around 2000 when shifting from P to R wouldn't explain why the entire car jerks... if idling around 2000 is normal, that is.

Harsh idle disappears once the engine is warm, and the shifting from gear to gear becomes somewhat less harsh. Is it possible that a jerk caused by a bad transmission or engine mount would be enhanced by the high idle RPMs?