It really shouldn't make much more than 1-2 mpg difference if that. When in 2wd you are only spinning the CV axles and spider gears on the front diff due to the left CVs fixed engagement in the spider gears. The carrier itself should stay put unless its a 99 or 00 where the front shaft turned all the time. The 2002s seemed to be the cream of the crop for the NBS. Right out of the box those 5.3 T motors ran strong and got great mileage for a full sized truck. My 99 all stock was an absolute sloth compared to my dads 02 of the exact same specs and even with 2000+ pounds in the bed his still ran better than mine.

The 4x4's are higher in the air, that's one small ding on fuel economy. Also, many 4x4 owners end up putting on wheel/tire packages with much much more rolling resistance, and that's another ding on fuel economy. A big one actually.

+1 on the motor breaking in. I just rolled over 14k on my 6.2 and have seen it go up 3-4 MPG from when I bought it. It's all about keeping your foot out of it though :)

^^^Nooooooo, it's your tune!! lol

I bet your mileage doesn't improve. Seriously.

The tune and upgrading the intake tube was a noticeable difference for sure.
Based on what??

My truck gets about 19-20 hwy 15 town. Im the winter its a diffrent story with 4x4 or auto 4x4 on then take away 2 mpg. 09 5.3l ccsb 4x4

12 city, 16-17 highway with my tuned 2011 CCSB 6.2 3.73 4x4 leveled on 33s with 13k miles.

Be just a little lighter on the foot and those numbers will go up :)

Not much. City yes, but I drove 2500 miles all highway at 75mph and averaged about 16.5mpg. Those tires really kill it.