you may get better mileage but it'll take you quite a while to get any payback.
What gears do you have right now? 4.10 or 3.73?
you'll get better mileage for free if you just slow down
Quick math if you're getting 15 and get a 10% improvement (not likely) for the $1500 for gears & setup (low price) fuel @ 4.40/gal currently costs you 29.3¢/mile
if mileage improved to 17 mpg (very unlikely) price drops to 25.9¢/mile
At the 3.9¢/mile decrease it'll take you 38k before you realize any savings.
Thanks for the advice, but looking at a 10% improvement vs. having to drive a Prius, I'm going for the gears! I'm not givin' up my truck. I'll slow down first. haha.