Just grabbed a used trans from a 98 GT. Looks to be rebuilt at some point and also had a clean Torque converter. Car has sat for months before it had power. 1st gear shifts at 4K RPM with whine noises. Fluids are filled up and that's not the issue. Any ideas?

My car is a 98 GT, top swapped with custom tune for the old trans I had. Shifted fine before the diff blew apart the case.