That's a bad year. '08 was the first year of the twin turbo 6.4, and they had a slew of problems. The new 6.7 is by far Fords best engine yet... and I do meen Fords. This is Fords design, not Navistars. The 06-07 6.0's were ok after they worked alot of the kinks out from the 05. They can still give you problems (mainly i.e., EGR cooler, oil coolers, injector wiring harness, and turbo problems to name a few). The 7.3 navistar was to some and still is looked at as a well respected engine. Very dependable and long lasting. I knew a guy that had a 94 7.3 with 350k on the clock when he traded up to an newer '02 7.3.I drive (or rather my wife drives) a 2008 F-250 SD. The best truck out there IMO...
except for the fact that it has left me broke down on the side of the road on too many occasions to count. Don't believe that you could give me another Ford.
Not sure I like the rest of the vehicle but the Cummins is a hard motor to beat. I wish they would put in a chevy. The Cummins has plenty of power and will last forever and gets good mileage if you have the 5.7 . the 6.9 I have not been as impressed with. Really cant go wrong with the duramax either.
What gearing, ?2004.5 GMC 2500HD. I love it! I seem to be the odd one here because I'm getting 21-23mpg hwy and 16mpg or so city. I've done a lot of work/mods to the engine and transmission though. Not because I've needed to but because I've wanted to.