Compare the temperature of the rear discs to the front discs after a short drive (presuming you know the front discs are not locking up.) I had a sticking rear brake on my Ford truck a couple weeks ago, could hear a scraping noise which aroused my suspicion of a brake problem; so I felt all four wheels after a drive with not much in the way of braking needed. Sure enough, one rear wheel was definitely warmer than the other three. Truck is configured with 4 drum brakes and feeling the drums rather than the wheels, the temperature difference was even more pronounced.
Whether possible on you Mustang, depends on the type of rear differential, but I can manually rotate my truckās rear wheels provided both are off the ground.