FNG, the short story is it depends on what gears you have in your rear now, what transmission you have and your engine.
I was going to install a 5" lift with 35" tires, but I have the 5.2 liter engine with a 3.55 rear end and the automatic transmission. If I didn't change my rear, the truck would have no power, my towing capability would suck, and my tranny would take some serious abuse. If you have a manual transmission and a higher gear ratio, you might not have to change your rear. I was going to change to 4.10's but doing so would require me to change the rear case. The dollars started to add up so I put the whole project on hold to save up some more dollars. Good luck man.