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