Yeah, I wouldn't mess with the diffs either. We did a transfer case removal to get lower gears in, and that was ok, but probably as far as I would want to get involved. Some people have the Diff done and then do the wiring/hoses themselves.
The suspension is easy. If you own a good set of sockets then you probably can handle it (nice test, eh?
) It's removing bolts and following instructions for the most part. There are also how-to's on many of the Xterra club boards.
Enjoy!