XJ is spring over and YJ is spring under (iirc), so there's one potential problem... perch to perch spacing is likely different as well. Good luck...
if I was her, I'd be looking for an 8.8 with 4.10s in it already (more common than XJs with 4.10s), swapping it in should be the same as doing one in an XJ (well, closer to doing one in an MJ I guess since it's SUA), it's definitely not bolt in though.
If you do go for one though, 4.10:1 is most common in 4 cylinder manual XJs. Try to shoot for an 8.25 instead of a d35 if you're getting an XJ axle and will have to customize anyways, since the 8.25 is a much stronger axle.
Are you sure you can't swap parts only and leave the housing? Sounds to me like shaft and maybe a spider gear are hosed, did she break the gear teeth or the carrier/bearings as well or something?