You have probably seen it but check out
http://www.hankshelper.com. Put all makes and all models under the search. In model keywords put "(Chevy|Chevrolet|GMC)". Then enter the distance from your zip you are willing to search and the years. It should search every craigslist in the area you indicate and put the results in a single list. I have been using this for about 2 weeks now and have found some promising trucks in a 200mi range but not ms.right.
Edit: You will have to filter through some car listings and z-71 listings but if you make it any more specific you can miss trucks.