I'm having similar issues. I had to tow a car on a dolly a few days ago and noticed my front brakes were getting real hot and my rear didn't seem to be doing much. Like you, my parking brake seems to be functioning well.
I was doing a search and this thread came up. I'm wondering if you found the problem with your truck.
Check your RPO codes to determine which brakes your truck was given.
RPO JB5 or JD5 is common on many C/K1500s, which are 10" leading/trailing brakes that generally do not stay in adjustment. The shoes are 9.96" x 2.24".
RPO JB6 or JD6 provides the 11" duo-servo brakes which are much better, with bigger diameter drums and wider shoes (11.15" x 2.75")
@Schurkey may weigh-in on the matter, he knows the brakes well and has posted much about the 10" and 11" drums, and other info.