That is called the rag joint. Replace the whole thing. This is a good excuse to jump up to an aftermarket shaft. Maximum Motorsports is every bit as good as Flaming River and $100 cheaper last time I checked.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.