Your shoes might be glazed. Someone or yourself could have driven your truck with the e=brake slightly on, heating them up and putting a glaze on them. Which in turn would make stopping, like trying to stop on ice.
My suggestion, since shoes are so cheap are to put in a new set, and sand down the drums the best you can to score them up and get better traction.
I would bet that will solve your problem.

The only other suggestion I have is that if you truck is a little older, I didn't read the year), the cable could be strechting and not applying full force to the rear brakes.

Just my .02 cents on what I would definetely do, instead of trying to figure out other ways. This way you know what has been done, and where you stand.
Chezman

PS Post back if it works.