Transfer case upgrade Question.

This is my first XJ and my first attempt at modifying a vehicle for trail/daily driving use.

I have a '99 XJ 4.0 I6 Manual trans. ax-15 (I think), stock as a rock. I have a 4.5" RE Lift ready to install and have been buying other parts in preparation for installation. I am going to go tomorrow to look at a NP231 with an Advanced Adapters SYE kit installed.

My question is will an early 90's np321 work in my 99xj, manual Transmission? What should I look for when buying a used np231?

Thanks in advance for all your help. I have been reading these forums religiously for several months now and have gained lots of knowledge it is time I put it to work. With the purchase of the newly found np231 i hope to get to work on it next weekend.
Half Hitch

Just make sure the t-case is a 23 spline input. Pre 91 and 4cyl jeeps got 21 spline. There are two different input lengths between cases behind autos and manuals, but I've put an auto t-case behind my manual without issue.

Pull the drain/refill plugs and stick your finger in there to check the amount of play in the chain. Just makes sure it isn't to sloppy and moving all around. Check the color of the fluid in it to make sure that looks good. Just overall inspect the case for any cracks or damage and such too.