SYE with Ford9" rear

I have an 91 XJ with ford 9 in the rear. I do not currently have an SYE but do have a longer driveshaft with no issues on 37"s and 8 inches of lift or so. I want to put in an SYE while i replace the leaking TC seals. How would i know if i need a new driveshaft? Any insight would be appreciated.

First off, not all sye's are the same length. You will almost certainly need a different length driveshaft, but more important, you will want a drive shaft with a dual cardan joint instead of a single joint at the t-case end.

Normally, people install the sye, then measure the distance at ride height and take that to the driveline shop. Whether you modify your old shaft or have a new one made is one of cost. Once you measure the length, you might get lucky and find a shaft the right length out of some other vehicle.

What he said.. going from slip yoke to slip yoke eliminators you DEFINITELY need a different style drive shaft.double cardan like the front. A lot of people run stock front driveshafts in the rear after SYE. Yrmv at 8” lift... that’s not the norm round here for sure

Since yours is a 91, I think you need the Hack-n-tap kit with the seal. See Iron Rock Offroad. Others sell it too. There are instructions on how to measure and then cut the TC output shaft. So, a front driveshaft would work in the rear. There is also the custom rear driveshaft option. I had to cut mine at the minimum spline length of 1-1/8" to work on my 4.5" lift. I some ways I think this helps in not having extra length output shaft to wobble.