Look, fitting gears into a rear axle can be learned and done, IF YOU ARE WILLING TO APPLY YOURSELF TO THE TASK.
Most people are not willing, and have problems with other, more simple procedures, like swaping out a motor or tranny. Hence, what MBrooks said (if you have to ask...).
Can you learn to do it? Of course, it is just mechanics, but ask yourself a couple questions first.
1. Am I willing to purchase or rent the special tools I need to do this job?
That will mean having a dial indicator, a couple of GOOD torque wrenches (dial type in inch and foot pound ratings), a heavy duty tool set that can deal with things like pinion nuts, etc., sometimes a case spreader, a bearing puller (or at least access to one), a dial caliper or micrometer, a stock of shims, bearings, crush sleeves, and a GOOD manual that details the steps to setting up the gears and getting them right.
2. Am I willing to LEARN how to actually set up gears?
That means studying the way that gears work, how they align, what you have to do to move them incrimentally to set them up properly, and how to be able to tell when they are correct. This will take some time, and perhaps even practice on a junk rear end (which is the best way to learn how everything works).
3. Do I have the patience to set-up, measure, mark, and then tear down a rear axle multiple times until it is perfect?
That means that almost no one gets the gear settings correct on the first try. You have to have a pinion pre-load, backlash, tooth pattern, etc., all RIGHT ON THE MONEY, no matter how many times you have to take it apart to get it that way - hence the main reason most shops charge what they do. It typicaly takes at least 3 tries to get it right, and sometimes I have seen as many as 10 tries until it is correct. Unfortunately, there is no hard and fast rule about how to get all the various settings correct the very first time. There are simply too many variables.
IF you are willing to go through the above steps, you CAN learn to set up gears and be the hero of the neighborhood and save yourself some money in the long haul (the first job will likely cost you almost as much as taking it to a shop becasue you have to procure all the tools, manuals, etc., listed above).
