I assume that on a truck with rear disk there is a star wheel adjuster on the parking brake shoes, it would be accessed through an oblong hole in the backing plate and it should have a plug in the hole. Jack up the rear of your truck and then remove the plug and use a screwdriver to turn the star wheel as you rotate the tire until you start to hear some contact. Reinstall the plug and do the other side. After this has been done check to see how well they hold. If the parking brake peddle still goes down quite a ways there is an adjuster under the front end of the bed on the drivers side were cable slack can be taken up.
Kevin