I was told that getting the lower gears (3.92) would allow the truck to switch into economy mode (go from 8 cyl to 4 cyl) easier and more often, as the load on the engine is lighter with the lower gears, so you actually would save fuel with the lower gears by being in 4 cyl mode more often. Can't vouch for that personally though. Just another factor to consider in all of this.
I've seen this as mine switches into ECO very often and has done it since 21 miles.