School me on replacing gears...

[Chrispbrown36]

This isn't a project I am planning anytime soon....just thinking ahead.
I have an 87 V10, TBI 350 auto with factory 3.08's and a locker. The gears are pretty lack-luster and to be honest I haven't paid attention to whether it really helps with the gas mileage...first of all because it's a new truck to me so gas mileage takes a back seat to just driving my new toy and second because the only thing I have to compare it to was an 86 2wd with unknown gears and a 305 in it that I had around 13 years ago. I do think that the motor needs some work to get it to perform better, but i really think the gears are dragging it down too.
Anyway, as I said school me on gears. I keep reading that 3.42's seem to be the sweet spot with these trucks, but can I switch out the 3.08's piece for piece or will I have to change some things up? Also with the factory locker will I need to put a new one in or can I keep the original in place with the new gears? Anything else I need to know? Brands? Prices? What about having a shop do it....prices on that?
Thanks in advance.