You probablly have 4.10:1 gear ratio which is commonly referred to as 4.10's.
If you have a 4wd truck the amount of money spent on fuel would have to be quite large in order to recoup from the swap.
I say this because your looking at about 400-500 per gear set then another 500 for the installation. So on a 4wd drive truck it would cost you approximately 1500 dollars to swap out gears.
In a 2wd drive truck it would be about 1,000 dollars.
If you tow with your truck often your better off to keep the 4.10's gears you currently have. As you get better acceleration with the 4.10's.