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
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.