The rear brakes on my 2000 GMC Jimmy are disc brakes, not drums. I just changed the rear brake pads and rotors at 83,000 miles. The car is currently at 91,000. Grinding noise at low speed but the strangest thing is that the outer surface of the rotor is smooth whereas the inner surface is rough. Plenty of pad left both inside and outside. The difference in surface texture on the same rotor really has me baffled.
It appears you know what the grinding noise is, but you really don't ask the question of what's going on. I'd suggest one of the slides in the caliper bracket isn't moving freely or that one side of the brake pad in the rear isn't moving freely. You are going to have to take it back apart to discover exactly what is going on.