I have a very weird problem with my car...

The problem is not consistent...but the rear end of my car seems to shake and jerk around a bit. When I'm at a stop, it seems that the rear end jerks a bit...once every few seconds or so. As I'm accelerating slowly, it seems like it's doing it a lot more violently (it sounds like the car is about to fall apart), once I'm up to about 30-35mph, it doesn't do it at all.

It's ONLY in the rear of the car, not in the front at all... I tried shifting into 1st or 2nd manually and it didnt do anything to help the problem...the problem also happens in Reverse...though it goes away when I'm in Park. (Haven't tried Neutral)

Any ideas as to what this might be? It sounds a lot like a transmission problem to me...I'm kinda hoping it's a rear end problem though, since I'm doing a rear end swap on Friday

Thanks guys...