If you find one with a slip yoke with no play, it is usable, and if it has
other issues, is reparable. The exterior appearance, even with a lot of
rust, does not matter.
The seal on the slip yoke (that has a zerk) can be replaced cheaply. Morris4x4
has it for about $7.
If the u-joints are bad, you can get original-quality Spicer 1310-X non-greaseable for
about $18 each. If you want greaseable ones, 1310-1X with the zerk in a cap, for a
few dollars more. Spicer 153X are the older, cheaper greaseable line and are OK.
The center yoke and ball can be replaced. Shafts up until 94 (or 95, I may be
wrong) XJs used a greaseable center yoke. Later XJs had a non-greaseable one.
There are quality rebuild kits available here very reasonable:
Make sure you verify before ordering whether you want it for a greaseable or
a non-greaseable centering yoke.
Maybe this will help.