Hi John. It's quite possible that the star wheel on the inside of the backing plate has broken loose from the stud of the bolt adjusters. I've had that happen a couple of times, mostly caused by the adjusting stud on the wheel cylinder being frozen up.
Are they all doing that? Have you successfully adjusted the rear brakes on that truck before? As Virginafireturck says the rear wheel you are adjusting needs to be jacked up off the ground a little so you can turn the wheel. Your post says that the drum won't release, does this mean they won't turn or they won't pull off the axle? Also make sure the parking brake is released.
Hope that helps, Paul.
