Hi,
Not sure if this is the same parking brake set-up as on the f-250 and f-350 chassis, but if it is, then it's an easy job. It is easiest to do the adjustment if you put the rear of the truck up on jack stands or blocks and take the wheels off but you can do it with the truck on the ground and the wheels on; it's just a little harder to test to see if you have adjusted the brakes too much or too little.
Directly behind both rear disks is a drum parking brake set-up. If you crawl under and look at the back of these drums you will see a small oblong rubber plug about an inch to an inch and a half long. This is the plug to the brake adjustment access hole. Use a large flat end screwdriver to pop out this plug and using a flashlight you will see a notched adjustment wheel. Use the same large screwdriver and move the adjustment wheel a few clicks (5 or 6) clockwise to tighten the brakes. If you have the rear of the truck up on stands you can spin the rear wheels and continue to adjust until the brake shoes start to rub on the drum and then back the adjustment wheel back off one or two clicks. If you have left the wheels on and the truck on the ground, you will need to start the truck, drop it in drive without Appling the parking brake and see if creeps forward then keep adjusting until it dose not creep in drive anymore and then back off the adjuster one or tow clicks.
I hope that helps and wasn't too convoluted.
Best regards, husk-her