Rear seal. There is a puller you need to do it and it can be done with the trans in the car. Unsure how much the puller costs but I know you can borrow/rent it from a user on the corral, just forget his SN.
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 ****.