If you change the pinion gear you should also change the crown gear. The crown and pinion are a matched set and should not be mixed up. even if you are installing a used pinion gear of the same tooth count as your original you should change the crown gear as well. They develop a wipe pattern from running together and miss matching them can cause whinning or a generally noisey rear end. I don't know how mechanically inclined you are or what tools you own but it is not difficult if you have the right manual to guide you through the process. Bearing "pre load" and " gear pattern" are measurments that are critical for success and unless you know how to set it up it's an invite for dissaster.
...really hard.....most tranmission shops wont even try it....its real hard to get the gears to mesh perfectly (and it must be perfect)....i saw 'em do it on tv...they put some kinda colored "stuff" in it , rolled it a couple times, looked @ the wear pattern...an diagnosed it..adjusted it...etc... i think you just about have to have a "new" chunk and pinion for it to be done (right).....s