Not all that difficult, but time consuming.
Yes.
Gotta drain the diff, pull the cover, pull the center pin & spacer, remove the c-clip, pull the axle shaft out. Remove old seal, clean up the end of the housing, carefully insert the new seal. CAREFULLY re-install shaft, lubing the seal surface first. Not difficult, but time consuming.
I'd buy 2, just because it would really suck to screw up the first time and have to wait for the dealer to get another just because you were too cheap to pony up another $7. It's always good to have a spare. In fact, for the effort, buy 3 and do both sides since you've done most of the work for the other one anyway.