Just read the last 3 pages, let me weigh in.
Basically I have done a lift, wheels, tires, cam and now gears to my truck. I religiously check my mpg every tank and average 12.5 with 50/50 fwy/city driving, through all of this, when it was stock on 265s, lifted on 285s and now on 20x12s and 33x12.50s, still the same mpg. Only thing that actually increased my mileage was going to 4.56 gears with these heavy 20x12s gained a little on the fwy, but lost a little in town.
That being said, I would start by getting some stock 17s or 18s because even oem 20s are way heavier, and buy a mild a/t tire setup around 32" tall and 265/285 width, that will be your least rolling resistance, best chance for an mpg improvement. Lowering the truck will have very little to do on mpg, because well it's a 5500lb truck and it takes fuel to move regardless of how high it is. If you're really worried about it get a hard tonneu cover as well, Tony has one and they make a big difference on mpg.
What Sam and Bryan are telling you is not wrong. I realize you have seen the higher mpg before, but don't expect it to do any better than 18 really, it still weighs 5500-6000 lbs.
Also I have done the math before on buying a commuter, and you can buy a hell of a lot of gas for 3-4k, and even then doesn't factor in maintenance or anything. I WOULD NOT remove your cv shafts because then what was the point of looking for any buying a 4wd truck if you can't use it when you need it?
I feel what is really killing you here is having a truck payment on it, and I would focus on paying the truck off as soon as possible.
