I just installed the 4.88 Yukon gears in my 05 F150 FX4. I have 35" tall procomp Xterrain tires, and went from the stock 3.73 to 4.88's and it works great. Gas mileage was killing me because I was having problems getting going from a dead stop. I used to have to downshift from 4th to 3rd on the highway going up hills. I can now stay in 4th gear and at 55 mph I am turning 1850 RPM. I also installed a S&B cold air intake, and a Edge Evolution programer. Everything works great.
I do all my own work on my vehicles, and the gear change took me about 5 hours on the rear. I plan on changing the front gears this weekend, I think the front will take me longer due to the IFS, and rack and pinion, and my 6" lift (crossmember clearance may be an issue). I hate working on gears due to one thing, the smell of gear oil no matter if new or used is just the most funky smell ever. If you get this on your clothers, don't wash them, just throw them away. My friend calls gear oil the black curtain of death, because when it comes out of the diffrential it looks like a curtain (because its thick) and smells terrible
I only ran into one problem while changing the gears, and that was the cross pin would not go back in because it was hitting one of the teat on the new gear (ring gear to thick). This is nothing a dremel would not fix though, I just shaved that tooth area down a bit. Apperantly there are no new cross pins for this issue, at least thats what Randys Ring And Pinion told me.
If you plan on changing your gears, I recomend Yukon gears, they make no noise as most aftermarket gears do. I hear no difference between the stock 3.73's and the new 4.88's. Here is a web site someone on this forum sent to me. It really helped me to pick the gears I wated to install.

http://www.4lo.com/calc/geartable.htm