I have noticed a "thunk" sound on the rear end just after it switches gears when I am accelerating from a stop. It usually only happens on the first and second gear. I took it to the mechanic today and he checked the U-joint and tranny mount and everything was perfect. I've got no clue what it could be now. Any ideas? It is a 4-speed automatic transmission.

Hmmm well that ruled out my first guess being the Tranny Mount. Now i'll throw out a few suggestions (guys correct me if any of them are wrong) but you could try checking your motor mounts. It really sounds to me like your driveline is overflexing and hitting the underbody of the car, and if your tranny mount is okay then i would check the other end and make sure the motor mounts are good. If its not those, check for torn bushings where your torque arm is, and your lower control arms. Nothing else comes to mind on what it could be besides these things...

I still bet the mount is broke. They always "look" ok untill the cross member is dropped.

Yeah that could be true. Those mounts are really compressed when the crossmember is still on, you may have to take it off to really see what you're dealing with.

*enter faithful biga$$ screw driver to be used as a wedge