oh, and car-part.com can help with your junkyard searches.
also, also, you'll want to remember the year of the axle you buy, as the brakes/bearings/etc changed over the years
for your purposes you'd be missing the long rod that connects them and it might turn into a 2-man operation to slide the seat. or you can finagle something up.
the number can vary, but not more than .02
for example: 4.09/4.10/4.11 are all essentially the same. the number comes from dividing the teeth and those can vary a bit depending on axle size.
3.54/3.55 is the most common XJ ratio out there so it should be easy to find. 95-99 are the best
My t-case skid is 1/4" plate that I had the steel shop bend up 1" on each side (I needed one made for my control arm crossmember and it was cheap to have a second one made up). not sure if that works for a stock t-case though, as my trans/tcase is from a wrangler and so is up a bit higher.
I finally found a stock fuel tank metal skid that will eventually be going on.