i have a couple questions
1) what did the backing plates look like where the shoes rub/contact at the plate?
there should be small pad areas where the edges of the shoes rub on the plate, reason i ask is they will wear at those points and get grooves worn in them, these grooves can cause the shoes to hang up and not apply correctly. This is a common problem especially in these trucks that seems to get overlooked quite a bit.
2) what did the shoes look like? were they clean and dry? no grease or brake fluid embedded in them? If they had gotten any grease or fluid in them at any point in the past they should be discarded and new shoes installed. Once they get contaminated it is impossible to get all of the grease/fluid out of them and it will seep back out when they get hot and cause them to not grab properly.
3) also the drums, is the contact area good? no wear/grooves?

I know you don't want to take the hubs back off to inspect this but you may have to.