Your truck has drum brakes in the rear. Several things can cause the brakes to lock up on"one side". Contaminated brake linings due to a brake hydraulic system leak, or grease or oil from the wheel bearing or rear differential leaking past a seal can contaminate the rear brake linings. Another possible cause is the brake shoes may not be properly returning due to a bad brake return spring or seizing emergency brake cable or linkage or a wheel cylinder may have failed. In any case the rear brakes need to be inspected to determine what is wrong and retify the problem. If both rear wheel prematurely lock up the brake hydraulic proportioning valve may be the problem but an inspection is necessary.