its not hard at all, but it is something you need to be very careful with because it can be really bad when it goes wrong. case in point i recently had a diff i built for a hmmwv go bad on me while on a road test. it turned out not to be anything i did wrong, what happened was the goober that put the diff's into the truck forgot to put fluid in it. they handed the truck to me to do a final inspection/road test and about 11 miles down the road the front diff completely locked up on me while doing 45mph down a country road. i still have the r&p on my tool box, i'll post up pics once i take them. the thing is, if you put it together too tight then it could lock up on you even with fluid. if you build it too loose then you can run into problems with the gears teeth bashing into eachother too much and the diff won't last nearly as long as you want it to, even worse a tooth could break off and then you are in deep doodoo. you also have to worry about the bearing pre-load on both the pinion bearing and carrier bearings. if they don't have enough preload or too much preload you run the risk of the bearing failing prematurely.
so while the process itself isn't that hard it does require a lot of care and attention to detail. you'll also want some special tools like a beam style torque wrench that measures inch/pounds, caliper, dial indicator, and case spreader. you'll also need some prussian blue, its a special paint you use to "read" the bear mesh after you're done. you then also need to know how to read the markings. you will also need tools for removing and installing the bearings and races.