A friend of mine has a 2005 Duramax that he picked up for a ridiculous price due to someone getting a divorce. He has a company truck so it spends most of it's time sitting and a lot of parts on that truck need to and had to be replaced.

That and maintenance, at they very least they tend to do oil changes and if something breaks it gets fixed.

My 2001 Excursion bought new with 50 miles on it now has 422K on it and is on a Ford re-man trans that has survived almost 10 years and 300K so far. This truck saw a lot of highway miles and I maintained the crap out of it when it was my daily driver. Oil changes every 5K sometimes less, Trans flushes every 30K, and whatever it needed it got. People shocked that it's still on the original engine and still looks as good as it does.

The same goes for the 2005 Excursion...I bought it with 123K 3 years ago this year and it's about to roll over 205K on it. I'm not scared to drive my 6.0 anywhere and almost always do.

Both pull the heaviest trailers easily when hitched up and neither really breaks a sweat while being worked.