If only one rear wheel is locking up the problem most likely lies with that wheel. I had this problem with my truck, the right rear would lock up on a gravel road. Adjusted the brakes as closely as possible, it would still lock up. From side to side I swapped the drums, then the shoes and finally the return springs. When I swapped the return springs the left rear would lock up. Those springs were weak and when pressure was applied would allow the shoes to contact the drum before the other side.
John
Milford, MI
