You don't give a year for the XJ, so this may be a little off.  I did the
conversion on my 88 MJ when I swapped out the BA10/5 5-spd for an AX15 so
some of the parts I needed won't apply.

First, even my 88 bellhousing would have worked.  I got the works from a 94
YJ so I had the bellhousing but comparing the two showed that I needed the
pivot ball from the newer one (hole already there) and I needed to tap
threads in two holes not used with the internal setup - already drilled but
not tapped.  You may not be as lucky.

Not hat it is essential, but swap out the clutch and pressure plate while
you're at it - save a second trip in a short while. Might as well change
the pilot bearing as well - cheap part, probably could stand a new one.
Inside the bell housing - get the whole thing if you can just to avoid
hassles - I would just go ahead and replace the throwout bearing - based on
the same logic as the clutch.  Watch out for one gotcha: your transmission
front bearing retainer has a long nose that serves as a slide for the
throwout bearing.  Check the diameter of the nose piece and make sure it
matches the center of the throwout.

The external setup uses a different master cylinder and the connectors are
different.  The internal setup has threaded connectors on the hydraulics
where the newer external slaves have push fit connectors with locking pins.
For a conversion, you might as well buy the pre-loaded set of master and
slave.  In the 88, I had a problem with two things:  The mounting holes for
the master cylinder were just enough different that I had to drill them
out - but your's may fit just fine as I was on the older XJ/MJ firewall.
The other problem I had may also be similar - the YJ pushrod from the pedal
was a different length so I had to fab an adjustable rod.

If you do change this, make sure you grease the nose piece that the throwout
slides on when you assemble things - a light coat of chassis lube will
suffice - so that you don't wear grooves on the shaft.