Couple of gear/rpm/mph calculators. You can find more just by using Google and those terms.
http://www.wallaceracing.com/gear-speed.php
http://spicerparts.com/calculators/t...rpm-calculator
According to following page it seems I was right about there being different housings for different gears.
http://www.drivetrainspecialists.com...-12bolt-truck/
Series 2 takes 2.73 or lower numerically, series 3 for up to 3.42, series 4 for 3.73 and up. That site also lists lot of different Limited slip differentials. GM uses Eaton so that's an option.
While it is something you can do yourself, personally I'd have it done. There's a crush sleeve on the pinion that needs to be torqued to something over 100ftlbs if I recall correctly. Then there's the gear backlash that needs to be set with shims and measured with a micrometer. The backlash will change if you change to a new differential or new gears. After that's done you need to check the wear pattern. It looks fairly simple on videos, but it's probably not that easy and you need some more specialty tools. Probably better off ripping it out of the truck (so they dont charge you to do it) and dropping it to a pro to do correctly.