Something loose back there or shot bushings or rear not properly aligned etc... Gotta get it up on a lift and really check it out. Or pull it, do a FTBR kit, replace any wear items and pop it back into place. Will feel like a brand new car.
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 ****.