Anyone changed their rear end for more towing capacity?

Changing gear ratios will not void your powertrain warranty as long as you stick with a ratio offered by the factory. Since warranty coverage is a concern, do not use an aftermarket programmer to adjust the speedo. Dealer now look for that if anything goes wrong. Instead, either go to the dealer or any independent repair shop with a "GM TECH-2" terminal and they can do the programing, if it even needs it (many trucks use the rear diff speed sensor, which won't be affected by a ratio change).