I'm following up on a thread I started a few weeks ago about installing a NP231 transfer case from a TJ into a YJ. I installed the case this weekend and am pleased to report that, for the most part, its a straightforward replacement. In this particular case, I replaced a stock NP231 in my 92 Wrangler (4.0L manual) with one from a 1999 TJ Sahara (23 spline). The transfer case is low-mileage (14k) and already has a Teraflex SYE installed; I picked it up for $375 on eBay.
The SYE requires a CV driveshaft, so I can't comment on whether the TJ slip-yoke will work with a standard driveshaft. I'm also going to install a PosiLok (bought, to be installed in 2 weeks) to eliminate the vaccum axle disconnect, so that gets around the electrical/vacuum switch difference in these two models. What I can comment on is the shift linkage difference. It's a great fit as long as you use the entire YJ linkage, all the way to the connecting nut on the transfer case. The TJ linkage is located differently and one of the ways this is accomplished is by using a connector at the case that angles the linkage to position it correctly. Unless you want to do whatever floor cutting necessary to make the TJ shifter fit, use the entire YJ linkage.
So, YJ owners have a good source of replacement transfer cases in TJ models.
Dave