Welcome....
To change either the trans or rear axle , you'll need to change both, and loose the torque tube..
Gear ratios will depend heavily on your entire drive train..
Trans final ratio, desired speed, desired rpm at that speed, and tire size..
Heres a rpm calc to help you figure it out..
http://www.csgnetwork.com/multirpmcalc.html
And we like pics...show us your truck.....