I don;t think there is any benefit to using a ZJ rear diff, and especialy if you're thinking the D44 that they have... forget it. IIRC there is too much aluminum in it and nothing really fits it....
Look around in the boneyards for the XJ D44 as those are still in existance and that's your best bet if you're looking for an easy swap. If not... then the 8.8 out of a Ford is a good setup.
Kejtar