To the OP, I don't know why I didn't realize you were in Missouri. Yep, all used cars in Missouri are tail-light warranty, no lemon law applies.
That said, buying a used trans and swapping in the driveway isn't super hard, but the computer-interface stuff is complicated and sometimes the dealer has to flash the PCM to accept the new unit.
First things first
I'd take it to the Mopar dealer near you, and have them run diagnostics on the trans, it could be something as simple as a bad solenoid, and the car is locking you out to prevent damage. It's highly likely that the used car dealer can't access this functionality in the transmission computer.