Ok, I have read all of the above. Rockola gives a little bit better empirical data, but let me give you a little bit more data to put it into perspective, because as everyone said, if the person beats the crap out of the vehicle, your gonna break stuff.
These vehicles are pickup trucks with a different body on it. My friends who are contracts run fleets of these. They beat the crap out of them, run them to 350k, see how much is left on the tranny, 99% run till 450 before they dump them Look e-bay, you will see these with 500k. The transmission is the weak link. The 4l60E is for all purposes not repairable. Every single part has been upgraded significantly. My tranny guy see's these usually around 115k, mine went on my 2000 5.3 at 180, my 2004 Subby is at 210 on the original. No explanation. Built on Wednesday. Obviously don't go to AAMCO for tranny work, shop it. I had a walk out of the door price for $1900 for total rebuild, 3 year 50k warranty. I upgraded it the 4L65E, which the Denali's have, just use's Planetary gear which is heavier. When my tranny was taken apart on the 5.3, I tow a lot. There was zero wear because of towing, tranny guy said towing doesn't make a difference. Tranny's are made to tow 100% of the time. It's luck of the draw. I flushed my tranny at 30k, asked him if it made a differnce, he said it's a 4L60E, who knows.
The 5.3 has been around for a long time, and its pretty bullet proof. One thing about this truck is it is was to work on yourself. There is a Youtube for everything. If you take care of the truck, it will take care of you. Too many people unload these at 125k because it nickel and dimes them. Then they go and buy a new one for lot more and it costs them more. Then people like me buy them cheap, $750 and I am good for another 200k, as I use upgraded parts.
If you are in the north, these are notorious for cancer in the front of the rear wheel well. Fix it early. Don't bondo it. They make the replacement panel.
Like many, I am a fan of the 200-2006 vintage. Best years they made these, plus they have steel bumpers so you don't scratch your bumper when you run over a prius.
Hope that helps..