I did this exact swap ( XJ D44 -> 98 ZJ ). Found a stripped down ZJ D44 with the aluminum housing. Had National swap the brackets from the ZJ D44 to the XJ D44. They did a decent job, but a machinist may have wanted it more perfect.
Didn't want to down grade to drum brakes so I bought a disk conversion kit from terraflex. There is likely a way to swap in the stock ZJ D44 disks, but I was looking for easy bolt-on.
Here are the issues I ran into:
- the terraflex kit requires that you install a ring spacer between the outer axle shaft bearing and the plate. It's difficult to get in place without using some silicone to hold it.
- the XJ D44 doesn't have an ABS tone ring. There may be a way to press on a new tone ring and fabricate a way to mount the ABS sensor, but I gave up on that and am looking to disable the rear ABS.
- the axle leaked out of the one side. I was told that the old XJ shaft was pitted and gear oil was seaping out. I ended up buying new superior axle shafts.
Lost track of how much money I spent, but it's roughly$2000 - $2500 including the gear swap.
If you're doing the fab and gear work yourself you could probably do it for $500 or so. Lots of labour though.
Hope that helps