Another one of the questions we get is about the many different types of NP231 transfer case for your Jeep. This list is merely based on the spline count and input shaft length by transmission.
There are other differences between year models of Jeeps. For instance, the NP231 in a YJ and TJ may have the same spline counts and input shaft length, but the shift linkage is inverted, making for a difficult install.
The YJ 231 also does not have a true neutral as the TJ 231 does, which would prevent flat towing with a YJ 231.

88-91 (Some 91s will have electronic speedometer, 91 was a changeover year)

AW4 auto/Peugeot BA 10/5 manual transmission/AX-5 manual/AX-4 manual
Cable driven speedometer
21-spline input
Short input shaft length - 1.2"

AX15 manual transmission
Cable driven speedometer
23-spline input
Short input shaft length - 1.2"

91+ (some 91s may have cable speedometer)

AX-5 manual/AX-4 manual
Electronic speedometer
21-spline input
Short input shaft length - 1.2"

AX15 manual transmission, 30RH/32RE automatic transmissions
Electronic speedometer
23-spline input
Short input shaft length - 1.2"

AW4/46RE automatic transmissions
Electronic speedometer
23-spline input
Medium input shaft length - 1.7"

NVG3550/NSG370 manual transmissions, TF999 automatic transmission
Electronic speedometer
23-spline input
Long input shaft length - 2.1"