I recently bought an 08 XL Denali variant with 83K miles. Never towed with. Dealer maintained from day 1. Two days after I brought it home, I did a complete transmission fluid flush and filter change (internal trans pump method), differential fluid change on both axles, transfer case fluid change, motor oil and filter change, throttle body cleaning, and replacement of spark plugs and wires with new OEM's.
The truck runs like it just rolled off the assembly line. I got lucky with a very clean 2008 6.2 liter that does not have cylinder deactivation. Just installed a brake controller and can't wait to start towing with it. I'm confident that this truck will be an asset for years to come. And the 6.2 vortec makes a nice rumble with my newly added intake tube.