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 #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.