I don't know what would make some mechanic more qualified than your average joe, but do this easy stuff yourself. There is more than one way to do this, but the first time you do this, do it the thorough way so that you understand what you are adjusting when you do this every time you change your oil.
Take the rear wheels off the truck, jackstands of course.
Pull off the rear drums, this might take some force.
Look at the bottom of the brake shoes and between them you will see a star wheel with threads out both ends. Adjust them some and slide the drum back on. I usually spread the shoes until the drums won't go on, then back it off a bit.
This isn't rocket science, and you will tell the difference when it's done. Don't forget to knock the dust out of the drums, and email me if you have more questions. I'll check it later tonight.