Depends. If you want FT4WD, get the NP242. I've also heard that the planetary gearset in the NP242 has more "planet" gears, which means that the load is spread a little more (and less stress on each geartooth, which contributes to overall longevity.)
If you don't need FT4WD, you can use the NP231. Both are chain-driven, and I'd prefer gears... If you get a used NP231 or NP242, I'd highly suggest you give it a going-through, if you didn't already plan to.
Either should work neatly with any transmission using the Dana 300/NP transfer case mounting pattern - and if you want to know more about the various transmission outputs, I'd suggest going to either the Novak Adapters or Advance Adapters sites - both are quite informative. You may want to get your transmission first, so you know what sort of output to match up to (there are a number of input gears available for the NP231, and probably for the NP242 as well - as well as other transfer cases.) It's a bigger job to change the transmission output than it is to change the transfer case input - since the entire transmission mainshaft would want changing anyhow.
I'm going to assume you have the AMC242/4.0L inline six - which means the easiest (and most direct) swaps would be either the AW4, AX-15, or NV3500. The AW4 is a fairly common box, but you'll need electronics if you don't have them already (if you have a 2WD AW4, you have all the electronics you need for the trasnmsission already in place, since they work the same internally.) The AX-15 and NV3500 will both bolt right up, but converting from the auto would take a small bit of work. However, any of those three would work handily with the NP231/NP242 (if you don't happen to get a transfer case with the transmission when you get it!) and can readily accept a Dana 300 (and other cases) with the proper input configuration - again, Novak or AA should be able to give you information on that.
Help any?
5-90