For a gearing ratio go here
http://www.4lo.com/calc/gearratio.htm it is a gearing calculator. Basically you put in your tire size, gearing ratios of your transmission (I always choose the final gear for a highway speed) and the rpms you want, then plug in different rear end ratios till you get the desired rpm for your speed (I try to keep around 2k rpms at 65 mph). For the transfer case ratio just put 1:1, that will even everything out for you.
As for a differential, it really depends on what rear end you have to give you any good options. Although most people tend to narrow it down to the true trac or trak loc rear diffs.
Personally I am planning on going with the true trac, it is a helix designed limited slip differential. Which is a lot more reliable and requires a lot less maintenance than a trak loc does. The reason that the trak loc may require more maintenance is that there are clutch plates in there that engage to improve traction when needed. Those clutch plates can wear out and require replacement. Whereas the true trac is pretty much an "install and forget" kind of system.
Of course all the information I have on differentials is for the Dana 60 I am going to install. I am not sure which differentials are offered in other rear ends.
Hope some of this is able to help you out.
