Hmmmm..... Sanford, ME..... spent a couple of weeks out there on business a year ago.
Gear installs are persnickety. It's a game of thousandths of an inch. It's not difficult if you have the right tools (press, dial indicator, bridge puller, etc.) but can be time consuming and frustrating.
If you're going to 4.10s, the easiest, and probably cheapest way, is like 87manche says. Go find yourself a pair of axles out of a 2.5l XJ with an auto. These generally have 4.10 gears (VERIFY THIS!!!!) Then it's just a matter of swapping the entire axle assembly instead of just the gears. In spite of the much larger parts, it's an easier and faster swap.
I'm also unclear if you're talking about new or used gears. A bunch of us here won't install used gears. Tom (old_man here) posted up a good bit of info on why used gears are harder to install and get right. Personally, I find that the cost of new vs. used gears is the minor part of the regearing cost.