We have a fleet of 11 2005 Chevy 1500s and a few of them have the same problem.

With the parking brake (rear disk rotors) fully depressed I can spin the rear tires with my hands (jack up in the air). I have not been able to find any kind of cable adjuster for the parking brake. Everywhere the cables connect all I have found was a simple connector, not an adjustment nut, flange, etc.

 

You have to remove the rotor. It has shoes inside with an adjuster similar to a drum brake adjuster.

 

I have a 2010 Exp 1500. I adjust the parking brake so there is a little friction when you slide the rotor off and on. even then, I find that the parking brake doesn't do a whole lot of good.