I'm one of those people who would do almost any job myself, just to avoid going to the shop, but I think this is a job best left to the pros. Most of the general mechanics I know wouldn't even do it, everyone recommended I go to a rear end specialty shop. I guess the reason is that special tools are required, and some steps, like setting gear lash, take a lot of experience to learn how to do right. To make it easier on the wallet, you can remove the "pumpkin" yourself easily, and just bring it to the shop.
UNLESS - if you get a pumpkin with 3.73 gears already installed, and you are just talking about swapping the pumpkins, then that is easy, definitely can be done yourself.