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...