Its the back pads, guaranteed. One might be cocked sideways and not releasing all the way. Pretty common actually. Pull the back tires off, pop off the calipers and redo everything.
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 ****.