IIRC 91+ is 23 spline. But I remember reading somewhere that there was a gray area in 91 when Mitsubishi was switching over to the 23 spline setup and depends on build date...I'd read in to that more though. The spline count refers to the output shaft that slides in to the tcase.
There are also two ratios for 2g transfer cases. So if you go and buy a 2g tcase make sure it's the correct one.
See below for tutorial.
Threaded Mode