I 9" is probably not the best choice for a yj. The pinion is very low. You need to have a SYE and only mild lift, or you will run into drive line angle issues.
A 44 is a better choice. Axle shaft wise, they are comparable....44 is 1 5/16" 30 spine and the 31 spline 9" is the same diameter....
Just something to think about....
There are A LOT of threads around here on 9" vs 44 vs 8.8 vs 60....a little searching and you can find a lot of info.