If thats the one you took out... My guess is the might have two dif input lengths causing them to bottom out on one another not sealing properly. Before i pulled it all apart, I would connect the breather correctly and run to under the hood, Power wash underside of XJ, and look for signs of where its leaking from. Without knowing the length of input gear, Its really hard to say where it leaking from at this point.
If indeed the shaft is the problem. There is no adapter to fix it. Correct length shaft is needed. Could also be a bad tcase input seal, or you damaged the trans output seal while installing the tcase.
What was the 231 out of? 94', Manual? Auto?
After really reading this that link again.... Looks like POST 90's with AW4 have a med shaft. (Your 242 has this.) If your 231 came out of a manual. It will have the "long" shaft.