Back when I bought my truck in 2008, I bought the Hypertech Power Programmer to recalibrate the speedometer due to the wheels being a different size. I actually never even used the thing. It has been sitting in my tool box all this time. Since i am selling the truck, i don't need it. I paid $450 for it. It is part number 32500. When i looked up different models and years on their site, it looks like it will work on all chevy trucks with the 5.3 V8 from 01 to 06, not 05 like i have in the title. Mine was a flex fuel 2005 Avalanche. Just to make sure it works on yours, check the site. Ill let it go for $260 shipped to your door.
http://www.hypertech.com/products-results.aspx
http://www.hypertech.com/products-results.aspx