I do not have much to say about this, I would just like to inquire on any tips I may need for swapping an NP231 to an NP242.

Are there any specific adapters needed?
Would I need to swap out either axle?
Is the control arm for the 231 compatible with the 242, or will I need to buy another?

I haven't done one and others who have will be along shortly but it is something I plan to do as soon as I find a 242. If I understand it is pretty "plug and play". You won't need to change axles for sure and I think you can even use the same driveshafts.

The thing to watch is the year of the NP242. They had different configurations over the years. I uploaded a Word doc that lays them out pretty well. I believe I need the late 23 spline w/external slip yoke, 1996-2001 XJ's for my '99.

Other than the transfer case, if you want it to look factory, you need to get the shifter bezel, the wiring to be able to wire up the "Full Time" light on the dash and the metal gate under the bezel that the shift lever operates on.

That's pretty much it... Bolts in, swap the bezel and gate, if you care, and update the wiring, assuming you get the proper one for your XJ.

I have a working NP242 that I will trade for an NP231 if anyone is interested.

If you care about having your cluster's "Full Time" and "Part Time" lights working, you need the proper wiring. Unfortunately, the wiring is slightly different between 97-99 and 2000-01. In 2000-01, there is a short separator that connects your transfer case to another wiring harness (I think the transmission harness). Since you have a 2000 with an NP231 from factory, there is wiring present for the Full-Time light in your main harness, but not in the separator that goes to your transfer case. Try to find an NP242 with its 4WD switch connector still on it, because even if you can't find a whole separator for it, you can connect the wire in your harness to the wire on the transfer case's connector and the light should work. The Full-Time wire is black with a white tracer, and the Part-Time wire is black with a red tracer. The transfer case works fine if you even just leave the lights unplugged, but I personally like to have them working.