When you swapped the trans, did you use the same or different OD?? Does the OD work as a 5th gear or does not appear to engage?

Slippage may be from the OD. Possible cause is solenoid that has some fluid in it and is not allowing complete release of OD and/or combined with weak actuating piston seals is self engaging at a lower speed then slipping when you slow down and then try to accelerate after dropping your speed to turn the corner.

Once you get back up to a speed where the OD pump is creating sufficient pressure to keep the OD engaged and the planetaries doing their thing.

Duane