I live in WI, so a truck isn't a truck without 4x4. With that said, you will probably lose more than what it states officially. The 2WD is rated at 25 and 4WD is 23. I started averaging around 18. I am now up to 19 - 19.5. I think it will hover around 20. Since I have the 3.55 rear end, more aggressive tires, crew cab. I probably lost a 2 - 3 mpg because of those options.
Personally if I were you, I would go 2WD for price and mpg and get the LSD rear end option. Not as good as 4WD but for a little off roading, it would help, maybe throw some weight in back before going off road, for more traction. Then add some more aggressive tires if needed.