It doesn't need a full rebuild in order to drive like normal, but it will need some parts.
Sounds like the input clutch piston/seal, it's a pretty common problem. The bandaid for it would be some Lucas stop slip and a Trans-go shift kit, it usually works really well.... but it might not eliminate the problem completely.
The replacement trans would have to be out of an 04+ GTP, the previous years of GTPs had a different valve body and controls, and the non supercharged have different gearing, differentials and axles.
Someone else on here will probably have a better idea of what your best choice will be.
Edit: you said it has a manual mode? Tap-shift? If so, you've got a Comp-G...
