All kinds of seals. and rings, and measurements, backlash, ratios and micrometers, shims, adjustments and all kinds of other crap that I dont know anything about. Thats why I dont think I'll ever attempt it myself. At least not without the help of someone who has done it before.
I read through an article in a 4wd magazine about how to do it and lost interest halfway through, lol.
Gearing is one of the few things that SHOULD be done at a shop with the right tools and knowledge.