I have a 94 YJ with a 4cyl. and 5 speed manual 21 spline NP231J transfer case. The Case is broke need a new one. I have two used ones for consideration. One is a '97 with 23 splines and the other is a '91 with 21 splines. Will either one work if I changed the 23 spline to the 21 spline. thanks for your help.
I am not an expert on Jeep NP231's but here is what I know. They came in many variations, especially of the input gear. As you noted there was 21 and 23 splines. However, there were 3 different stick out lengths on the input splines. And 2 different helix angles on the planetary gears. Apparently not all of these variations occurred with each other, but there are still many possibilities.
Anyway Novak is a great place to research stuff like this.
I will start using Metric only when the duodecimal system is adopted!!