I know on my 1 Ton, the rear brakes are much larger and on a really slippery surface, they could be a problem. Brakes weren't as advanced in those days. Proportioning valves do help but I have no idea if they make them for a drum/drum setup. On some mini vans, they use a variable proportioning valve that is controlled by the ride height in the rear. They work great when new but as the springs settle, they provide more rear braking because of the change in the spring height, which of course is not what was intended.
