I just checked the box for the Reese universal fit hitch, and the same one does fit both the XJ and the ZJ. However, the universal hitch is adjustable for width, so that's not a guarantee the frame rails are the same distance apart.
There was, however, a recent thread about gas tank skids in which I think I recall someone posting that a ZJ skid will fit the '97 and newer XJs with the plastic gas tanks. If this is correct, that's another strong hint that the frame rails are the same distance apart and the same hitch should also fit.