Bart, you already know that I think you are very sharp when it comes to driving. Heck, I don't think that there is any one who can hold a candle to you when you get behind the wheel. So, please take what I am going to say with a grain of salt.
If you are thinking about re powering the truck then I would seriously think about getting rid of the truck and looking for another. Perhaps a newer LS series truck would be in order. Still gas powered, better fuel mileage, and more power while towing. It will not be a diesel but, it may be enough for you to get the job done. Head out to a few local dealers with your current truck and drive everything on the lot that you think may work. Test drives are free and who knows, you may be convinced to keep your current rig.
If you are serious about re powering your truck then you may be ahead if you consider a LS power plant. Big blocks are the old stand by but, it sounds like fuel mileage is a concern and no big block will give you enough mileage. Towing, unloaded, and driving off a cliff will only get you 10 mpg.
I am a huge fan of any modern turbo Diesel. I feel as though any of the big three offer a great product especially if you are willing to part ways with the warranty and delete the emissions. In Utah it sounds like that may not be an option though.
LT.