The main differences between the 231 and 242 are the 242's full time option. When you put a 231 in 4wd hi or low the front and rear output shafts are tied together and if there is no wheel slippage between front and back you will end up binding. The 242 on the other hand has a differential similar to what your axles have in the pumpkins, it allows the front and rear output shafts to slip as necessary.
Now the YJ you had more than likely had a front vacum disconnect on the passenger side of the front axle, when you engaged 4wd in that the vacum motor in there would slide a lock and tie the two sides together. On the XJ the vacum disconnect it not there on your year, the axles are normal one piece axles so they spin all the time as does the front drive shaft whether you are in 2wd or 4wd, the only difference is that in 2wd the front drive shaft is not 'connected' in the transfer case till you select full time or 4wd hi/lo.
There are no locking hubs on yours unless someone added them aftermarket.
