the work itself isn't all that hard its just time consuming, precise, and requires a lot of patience, plus the tools themselves will cost as much as the labor to get them installed..
I would have a QUALIFIED shop do them, if you mess them up or just don't get them perfect you WILL have noisy gears and run the possibility of them jamming and locking up (worst case). If they end up being noisy you have to start all over again, if a pattern is worn into them though you most likely will never get them right again so you will need new gears. Set up gears are also something you should have as well as a press, if you have the guts then try it yourself but I sure as hell would be pissed if my pinion gear collided with the ring gear wrong at 60 mph locking up an axle and causing an accident, my xj is a DD though.