trans is slipping

ok, I have a 96Z28, and on full throttle the trans is slipping BAD, only when it tries to shift. Every gear does this, 1st is the only good one at full throttle, then when it tries to go into second the rpms just stay up and the trans slips. COuld this be a bad torque convertor, or just a bad trans. Somebody lemme know. Also if the trans is bad, does anyone have a trans for it.96 lt1. thanks

Matt