Your rear drum brakes may be applying little or no braking effort if they haven't been adjusted in a while.
Drum brakes need to be regularly adjusted as the brake shoes wear (every 1000 miles or so). Modern drum brakes are supposed to automatically adjust by coming to a full stop in reverse with the brake pedal depressed, but I haven't found that to work well. Your results may vary.
To manually adjust rear drum brakes, chock the front wheels, place the truck in neutral, raise the rear wheels off the ground and support the truck on jack stands (never trust a jack to hold a car or truck off the ground). Next find the access hole which gives you access to the adjuster star wheel within the drum assembly. It is located at the top of the inboard side of the drum assembly. Hopefully there is a plug of some kind to prevent dirt and water from getting into the drum assembly.
Inside is the adjuster star - a little wheel with teeth on it which you can turn using a standard screw driver. The adjuster star rotates on a horizontal axis parrallel to the frame rails and perpendicular to the axle and is like a ratchet wrench in that it only turns in one direction.
While turning one tire by hand, turn the adjuster wheel for that drum assembly until you feel a slight resistance. Then either live with the additional brake drag for a few hundred miles or turn the adjuster star back one or two clicks as descibed below.
Next to the adjuster star is a small lever which can be pushed with another standard screw driver. By pushing on that small lever, you can turn the adjuster star in the opposite direction to pull the brake shoes away from the drum.
Repeat the proceedure for the other wheel and go for a test drive.