Step 1) Verify problem, remove drums, put an indicator on hubs and turn them to check if they have run out or are bent etc.
You mention squeaky brakes, has it been since you purchased the truck or did it start later? Since there is 90% left, seems likely someone has changed shoes previously.
2) Are the shoes installed correctly like Snoots mentioned?
3) And what Bucket said about mating surface, I have seen this before several times in rust areas, is it just one side or both?
A Very small burr, ridge, piece of dirt or RUST etc. at the center of the hub where the first contact is can make a big difference a foot away. OR rust on the actual lugs and around the Center ring.
Solved it once by thoroughly wire brushing around every lug and center ring to a shine.
4) Did he replace any lugs?, are any broken, bent or binding?, or a different shape (as in a shoulder between the hub and the threads) where the drum meets the hub?
5) Check clearances, flip a drum 180 degrees to check if surfaces mate and lugs pass through drums smoothly.
Heck, even have the (hopefully not cheap Chinese) drums checked for run out, out of round, and casting errors too, new or not.
If the hubs are OK and straight, no run out or wobble, has All the rust been removed from hub?, especially where the lugs pass through the hub? reasonably reiterating.
Has anyone checked the emergency break? in case it was over adjusted, or from adjustments on pedal to cable, cable to frame where is it supported, cable to hub etc. (Squeak part)
6) Check the run out on the actual wheels, tires, and get a good balance job on them too.
Just half a dozen things to consider.
I am not a doctor, nor play one on TV, but would like to know the prognosis of said specific speculative situation surgically speaking sequestering maniacal metallurgy mechanical mess.
Jus' sayin'.
