The np207 that came with the 87 YJ looks very similar to a 231 but without the ID plate It is difficult to know for sure.
You can verify the gear ratio (as Crashnburn said) by shifting the case into 4 low and spinning the input shaft and counting the turns on the output shaft. (I believe that the 207 has a higher low ratio than the 231, by 0.11 turns on the output compared to the input, but I could be mistaken. I have a 207 on the floor of the shed that I could dig out if a pic would help). If it has a 4:1 kit you will know right away (4 tunes on the input for 1 turn on the output).
Last edited by Happy Joe; 07-18-2007 at 11:24 AM.