2006 Sierra wt, 2wd, ext cab, towing package, 5.3l, 4l80e, 96,000 miles. I bought it new. Oil changed religiously every 7,000 miles, always 5w-40 diesel synthetic, always pure one oil filters. Perhaps 10,000 miles towing a 3,500 pound trailer around the Rockies and another 5,000 miles with a motorcycle in the bed, otherwise just light cargo use. When towing I slow to 50-60 and keep the rpm around 3,000 - 3,500. The diesels fly by at 75 even while towing 10,000 pounds, but I don't got no stinkin' diesel.

In the last 5,000 miles I've replaced shocks (now Bilstein all around), tires, front rotors, calipers, brakes, inner and outer tie rods. I've also replaced trans fluid, radiator fluid, differential oil. My son and I did most of the work ourselves including the tie rods and alignment, total cost about $1,200 including tires and a bit of labor. The idea was to have the truck all ready for another 10 years / 50,000 miles, needing only gas and oil. (My son and I are both engineers, we did the alignment with 50' of string, a bit of cardboard, a good understanding of geometry, and a bit of TLC.)

It seems I can sell it for perhaps $17,000. I'm wondering if I would be smart to sell it and get a 50,000-80,000 mile tundra, or if the GMC is a long term keeper. The engine seems very solid, and the truck rides and handles quite well again. I have concerns about putting high mileage on the 4l80e; these transmissions only lasted 75,000 miles in the 70s and 80s, and it seems since about 2015 or so they only last about 100,000 miles. However I've gathered the late 90s and early 2000s were perhaps better made and lasted longer. (Why can't GM make an excellent transmission when they've been making the same 4l80e for 50 years?)

I paid $19,800 otd new, so getting $17,000 for it 18 years and 96,000 miles later seems like I got good service for my money. 

I see used tundras for sale with 300,000+ miles on them; it seems the 5.7 engine made before 2022 is simply indestructible. 

Keep the GMC for another 10 years / 50,000 miles or sell it and get a used tundra? 

Thanks, 

Mark