You either have the NP231 or the NP242. If you have a "Full-Time 4wd" selection then you have the 242, if you only have part time then it's the 231. On the 91 model you will also have the 231 for the vaccuum disconnect front axle.
You can also crawl under and look for the tag on the case itself. I can't remember exactly where it is, but it will tell you the model number and the gear ratio (which will be 2.72:1).
Either a 231(part time) or a 242 (full time). The shift pattern is different between the two but,I,m not sure of the name of the 4 wd full time.
A tag on the case itself will tell everything.
Wayne