Time for a new trans. Swap to a low mileage one out of an 04 which is an upgrade unless you already are an 04. Only thing you have to do to run it is have a tuner change the pulse wheel count in the ECM from 6 to 24.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.