2004 Pickup brake locking up

Are the rears drum or disc? First make sure you don't have more weight on one side of the bed than the other. If only one of the rears is locking instead of both together, you might have a brake fluid leak from the cylinder -- brakes dampened with brake fluid tend to be grabby. If they're drum brakes, check the self-adjusting mechanism (a complicated set of ratchets & levers) -- you want them to be clean & free-moving (lubed with just a dab of a grease that won't drip when hot). 'Course, there's always the possibility that one of the rear brakes is worn out and you're getting uneven braking between the working one & the worn-out one...