I am sorry to hear about your trouble. My GMC has been BY FAR the best truck I have owned to date and knock wood, no trouble after a bit over 20,000 miles so far. This is not to say that lemons are not out there and you must have gotten one. This really surprises me as these trucks have generally been trouble free. My advice would be to pursue retribution from GM by talking with their regional rep (if you have not already). In the past, sadly all the domestic makers, but especially GM, have been very unlikely to make right bad cars to their customers. One of my co-workers and friends had a top line Tahoe that was similar to your truck--an obvious lemon. It was constantly in the shop and had left his wife stranded on multiple occasions. GM refused to help him, even though the SUV was in warranty. His dealer would take it in, try to fix it, fail, and then get it back to him. He ultimately had to retain legal counsel to fix it but he did ultimately get a replacement vehicle of equivalent trim. As physicians both he and I have a disdain for lawyers but after trying to handle this on his own it became clear he needed one. Perhaps with the desire to maintain viability GM might be better to work with now on this, but I suspect they will not. You know that if they traded you out you would likely have a nice truck as they really are quite trouble free.
Not that Ford does better, but if you went that way I have experience with the V10 engine in the 250. I can tell you that they do not do THAT badly on fuel unless pulling big loads. They have plenty of power and are really quiet and refined. Mine, which I had about two years would get 14-15 on highway, unloaded, and about 8 pulling. My dad's 150 with a 5.4L and two wheel drive gets about that pulling a smaller load and does not have the same power as the V10 did, so I generally tell people the V10 gets about the same mileage as any gas truck/engine Ford sells. It is true with 2010-11 MY the 6.2L is coming out. Though new to the market, it is not a new engine to Ford. They have been running and testing this engine for a long time and given the modular concept the engine line has been well-proven in other applications. I do not worry about the 6.2L reliability--usually the electronics are the snafu early on. I personally would find a V10 truck if you wanted to get one now.
My question is how are you going to come out financially here?? If you "unload" a poorly running truck you could risk being held accountable to some degree in a private sale, and certainly a dealer would not be pleased to take a lemon on a trade. They also have ways of checking service records via computer and this might set off a red flag for them. Either way it might be best to FULLY exhaust the means you have with GM before dumping the truck entirely. I suppose the other thing is that this truck might have something fairly simple wrong with it that has defied explanation. You might request the regional service "targetshooter" be asked to make one final evaluation of the truck and they might have some answers. They key here I think is to not let the frustration of the situation dictate a move which might be financially better served by riding it out a bit more. How many miles on your truck??? Also bear in mind if you can document the truck was brought in while under warranty for a service issue which has persisted unresolved through the warranty duration, the company has some obligation to make it right even if you are out of warranty. I have several examples of GM dealers doing this on a personal level, thus a precedent has been set there. Good luck, keep us posted.
John M