I had a 85 k20 and the brakes just sucked too. I then found that the rear brakes werent working at all, and I found that the line from the M/C was broken and pinched off, then after I fixed that I found that one of the drums was full of grease. Brakes were real good after that.
The moral is that your rear brakes do more than you may realize, when those shops say they just need adjustment, what they are probably talking about is the star adjuster between the shoes. this little gizmo is supposed to adjust itslef every time you back up and hit the brakes. Now the problem is that if you've ever had the drums in the water or driven it in the rain or let it sit for a while, these things get stuck hard. Its also possible to get the shoes in backwards which still stops, but not as well.
Jack your truck up and have a friend hit the brakes while you turn the wheels by hand, just to make sure that they all work. If they do, rotate the rear wheels and see if theres any drag. if your brakes have just been adjusted there should be a little bit. If not, then you need to get the chilton manual out and pull the drums off and see whats up. Just look at the picture to see about the shoe orientation, then see if you can spin the star adjuster by hand. (move the little actuator out of the way, looks a little like a hockey stick and its left hand thread i think) if its stuck, then the shop who attempted to adjust it might have thought the adjustment was ok where it was. If you find it stuck, you can by a new one at the store or unstuck yours.
I really think every 4x4 I've ever owned has had improperly adjusted rear brakes due to stuck star adjusters.
Hope this helps you out.
