Bill – Hi, two ideas for things you can check. The drum brake is a simple device. When you pull the drum off it will be obvious if something is broken – it could be something odd like the lining coming loose from a brake shoe. The second idea is that the problem may not be in the brakes but inside the differential. There could be be something loose that locks the differential gear on the RR axle shaft when reversing. Does the locking occur when the car is in neutral?