Lous thump when accelerating

Hey guys, got a 94 Z28, automatic. The other day I noticed something start to happen. Whenever I really get on the gas from a stop, the car makes a loud "thump" when ever it shifts to 2nd gear. Usually it would chirp the tires but now it thumps. Doesnt seem to do it when Im taking it easy. Also, seems like the sound is coming from the rear of the car. Any suggestions before I take it to the dealer? Thanks.

could be the tranny mount, that is what mine was, good under and see if the mount is cracked or bad.

Could also be the torque arm mounting bracket(bracket is on the side of the tranny, torque arm comes from the diff.)

Also check the U-joints

Thanks guys, it was the Tranny mount.