RE: Gears
If you buy the whole center section, you can probably do it yourself. If you've never done it before, i wouldn't try to set the gears up myself.
I've never removed the differential, but from what i hear, it's not that tough. You have to get the car up on jack stands or a lift, lower the gas tank a few inches, remove the driveshaft, remove one rear wheel, disconnect the spindle at one end and swing it out of the way, remove the halfshaft, shift the center section to the side and pull it out.