When I put it in gear, I feel a big bang (kicks you in the *** ) when it takes off...at normal acceleration. Transmission has been rebuilt back to stock. What could this be caused by?

 

Missing checkball in the auxillary valvebody. Some people mod this and the aux accumulator for a firm shift into drive and reverse. Could also be caused by an aftermarket boost valve and blocked bias valve (for firmer shifts). Most people complain of mushy shifts.

Might not even be your tranny. Maybe your U-Joints are shot or your torque converter has too low a stall (if replaced) or your idle is too high.

 

I would check to see if the tranny mount is busted. If u say when u step on the gas u hear a bang then that is your tranny hitting the bottom of your car. Happened to me and the tranny mount was busted. Easily replaced....

 

If you have the 1982-1986 Transmission then the forward engagement is a little firmer, or if the idle is too high. Has the transmission been rebuilt? Has the problem always been there? I need a little more information to help out?. If you want give me a call Mon-Sat 11-7 PST at 800 7763288 and I will answer any questions you have.
Glad Too Help!!!