Here is my experience in converting my '89 XJ from a 5spd/NP231 to a 5spd/NP242 and eventually to a Auto/NP242.
The wiring harness that came with the 5spd does not support the fulltime light, some wire tracing and splicing would need to be done to get it to work. I nver bothered looking into it. When I swapped in the auto I swapped in the wiring harness and it suported the fulltime light, all I had to do was put a bulb in the dash. This makes me think that if you already have an auto you should be able to find the wire for the full-time light and splice the pigtail from your donor vehicle onto it. But your '98 is probably much different from my '89.
I too enjoy my NP242! Great for winter driving and also for those gravel roads on the way to the trail! Keeps the locked backend from kicking out on the corners. I originally swapped mine in because I can easily buy NP242s for $100 with low miles(nobody wants them!). Its cheaper than rebuilding my NP231. Used NP231s run well over $400 if they are in decent shape, $250 if the chain needs replacing. Much more demand for the Np231 from the YJ/TJ crowd drives up the price.
