I have a bit over 100,000 miles on my 05. Front ends are heavy so I put lifetime balljoints in it early, the u-joints in the front axle failed fairly early as well so I put the best, greasable ones in there and they've been good since.
Torque convertors can fail and the debris goes through the trans. so I had a new TCS convertor put in about 15,000 miles ago and I had a good trans guy freshen up a band and the front clutches at that time as well, it cost around $2k for all that but it should last another 150,000 miles now easy.
Cummins are good and durable but why go cheap on oil changes? I change it and the filters every 5,000 miles.
Air filter is important as well and I don't use aftermarket junk on my truck that will let dirt by, I use a stock type filter and change it often as well.
Engine is stock, sure it can make more power easily but the truck pulls fine as it is with a loaded, enclosed trailer and more power puts more stress on all of the drivetrain.
Truck gets 18-20MPG real world as well, loaded it can be a bit disappointing but that's because it is an automatic and rather than stay at 1500RPM and work up the hills where a diesel likes to work, the automatic kicks down and buzzes up at 1800-2000. It still gets around 12-16 loaded but would do better if you could hold it in a higher gear ( we've got 6%-10% grades here and they can run for 10 miles). Mileage picked up when I went to 285X70 tires but stayed the same when I went up to 315X70's.

Sheldon