If the repair is done at a dealer, they now use a revised Mustang drum, which includes a ridge to secure the bushing in place—likely the issue in your transmission. Be sure to replace the center support tower it rides on, as well as the outer planetary cylinder, which is probably worn out from the F-clutch friction rings. With updated seals throughout the transmission, it should shift like it’s brand new again.
I had mine repaired at around 85k, but the transmission failed again at 99k and needed a full rebuild. If there’s a way to prevent another failure, I’d love to know as well.