There was this thread
a couple of days ago, I list there a couple of articles and a GREAT video on gears change.
It doesn't appear to be too difficult if you have the right tools. I'm not changing the gears, but just replacing all seals and bearings, I got me a 12 ton shop press for this (and other stuff, hubs, bushing changes, etc) and it seems to be the culprit, together with a few other tools (bearing separators, 2/3 jaw puller, dial indicator, magnetic stand) you can probably get all the tools you need from Harbor Freight for under $150 (I did!)
If you're changing gears, then you need to calculate the new pinion depth, again, the video I talk about is a GREAT resource, just $30 delivered.