Some info from a cherokee forum
Transfer Cases
The transfer cases used are all chain driven with aluminum housings. NP stands for "New Process" which is the brand, if you will. The newer ones are NV instead which stands for "New Venture". It is the same transfer case just a different name.
NP207 - "Command-Trac" part-time only - 2.61:1 ratio low range - used 84-87
NP231 - "Command-Trac" part-time only - 2.72:1 ratio low range - shift pattern 2H - 4H - N - 4L - used 87-01
NP228/NP229 - "Selec-Trac" - 4Hi(full-time) - N - 4Lo (part-time) - used 84-87
NP242 - "Selec-Trac" part-time OR full-time - 2.72:1 ratio low range - shift pattern 2wd - 4part-time - 4full-time - N - 4LO - used 87-01