The rear brakes are drum type, there are no calipers to check back there…I suspect a brake shoe is dragging, (broken or weak return spring, partially stuck wheel cylinder, e-brake cable stuck). Far more likely than a differential problem. Pull the drums and make sure the brake shoes are returning to rest on their pins when they should be. Don’t be afraid to clean them up with a water hose or pressure washer so you can see thing better…