First of all I'm not an expert, I just had to make this swap, and wanted to pass on what I found. I hope it helps you.

The transfer case on my 87 Jeep Cherokee blew out (actually wore out due to loss of lubrication).

I tried to locate a replacement Selectrack NP-242 transfer case, but could not find one locally, and did not want to part with 800 bucks, so I decided to try a swap to a NP-231 Commandtrak which I know I could get used for $150.00

Some details:
The NP-242 is full time 4WD with the following ranges
2W High
4W High Differentiated
4W High Lock
4W Low Lock

The NP-231 is a part time 4WD unit with the following ranges:
2W High
4W High Locked
4W Low Locked
The NP-242 failed because the oil tube coming from the pickup sump to the pump fell out and got into the chain. With out lubrication, it burned itself up.
I will post some autopsy pictures when I get around to it.

The NP-231 has a different input spline than the NP-242, so I needed to swap the original input shaft from the old 242 into the 231.

Also the front output had a different yoke on it, I am currently running with only a rear drive shaft, I will take the yoke and front shaft to my local drive shaft shop and have him fix it. The rear drive shaft fit no problem.

Linkage: The shift pattern is different (Obviously), but I didn't expect it to be reversed. Yep when running in 2WD the selector lever in the cab needs to be in the up (pulled back and up) position. All the way forward is 4WD Low Lock. I'm sure I could correct this minor glitch if I felt like it, Maybe someday I will.

First you split the case open

This image shows what a pump, pickup tube and sump screen should look like.

This is the inside of the front half. On the right side is the planetary assembly that contains the input shaft. You need to remove it to get to the shaft.

This shows the two different planetary assemblys, on the left is the original NP-242 input and on the right is the one that came with the NP-231.

The inputs have a different number of splines too. The 242 had a 21 count spline, the 231 had a 23 spline.

Close up of the input coupler after being removed from the planetary gears.

This is the input side of the NP-231 with the NP-242 input shaft installed in it.

And the output side just for reference. (The tail shaft covers are not installed.)

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