If you are looking for good mileage, then stock is the way to be. I guess it really boils down to how much you are willing to pay to play! 4x4ing isn't a cheap hobby by any means...
Lifting your vehicle incurs more costs than just the loss in fuel economy. You need to factor in the higher costs of maintenance and more frequent break downs as well. The cost per mile to drive a lifted vehicle that is regularly run offroad is way higher than one that is just driven on the street. Everything from offroad tires wearing faster down to more frequent oil changes due to severe engine duty needs to be considered.
If you are looking for real world fuel economy figures for trucks of different lift / tire sizes, then that is a bit of a different story! I used to drive an old 1980 2wd 3/4 ton as my daily driver back in high school. That truck was bone stock, literally had nothing done to it but a painted intake manifold and valve covers. I used to get around 8 mpg with it driving it like a 16 year old. I now have an 85 with a 6.2L turbocharge diesel lifted 4" on 1 ton axles with 37" tires and can get around 18 mpg on the highway driving it like a responsible adult. That said, mileage doesn't mean squat to me. I put fuel in the tank once a year or so, put it on the trailer and drag it out to the trail so I am not risking other people's safety with how poorly my truck handles at speed.
There is really no magic number of what you'll get for mileage based solely on the size of your tires and your lift. The condition of your engine, how you drive it, and even your elevation will make a huge difference in overall mileage. I can promise you your truck at sea level gets better mileage than my truck at 15 000 ft above sea level in the mountains.
