I don't know about making the speedo work, but the NV241OR Rubicon T-case requires a 23 spline shaft on the transmision. I believe your 91 XJ has a 21 spline, they went to 23 spline in 92. I've read that some late year 91 also got the 23 spline shaft, but you will need to verify what you have. I'm running the 241OR in my 01 XJ with an AW4 Auto tranny and everything matched up including the speedo. You will have to deal with the driveshafts too. I replaced the front tcase yoke with a 1310 CV yoke so I could re-use my front driveshaft. I bought a new ds for the rear. The 241OR is quite a bit larger than the 231, if you are running a tcase skid you might have to modify it.