What everyone else said, go with the 3.92's the truck is hardly off idle at 60 mph with the 20" wheels.

 

IMHO, I would get the cheapest factory gears possible, then have a Detroit Tru-Trac and either aftermarket 3.92 or 4.10 gears professionally installed.

 

How much would it cost to have 3.92's professionally installed in a 4x4, and go to posi at the same time, I put a Richmond Gears Tru Trac or something like that in my 2wd 72 chevy 12bolt, man it was like I added HP since 2 wheels push now and they grab especially with the biggest size 15 inch BFG T/A's. Pretty wide.

 

It depends on a lot of factors. I should add that if you are considering buying a new 4x4, just opt for the 3.92's right away. It's a $375 dollar option, and you couldn't have the front regeared that cheap. Then if you want to upgrade the factory Trac-Loc have a Tru-Trac installed.

Cost wise on a truck you 4x4 you already own, you're looking at a minimum of $2,000 for a regear with front and rear LSD units.

 

'f' the 3.55, especially with the 20" stock rims, get the 3.92. my last truck was an 06 F150 and it cost a total of $2400 to regear front, back, and the full rebuild kit with Yukon gears from a local shop. That is one expensive upgrade after the fact. for a truck, the QC hauls a$$ with simple tuner/intake/exhaust do it yourself bolt ons with this gear setup. yes, I am liking my daily driver here..