Now correct me if I'm wrong (which has happened before...once or twice...) but on an older XJ (I believe pre-91) the front axle is disconected entirely when not in 4wd - so as long as you don't have a spool you're fine. On a newer XJ, the front axle is turning, but is not receiving any power when in 2wd, so an "automatic locker" like an Aussie that locks only when it senses torque being applied to it, would work - but this type would not work with a full-time 4wd system because it would try to lock whenever you applied the gas because it is receiving torque.
So if all that is true, then you're only choice for your jeep would be a selectable or a limited-slip (Just like CheapXJ already said)...
I say this all as more a question than an answer, because I have no personal experience with it, but have only read about it... Is that right? Could a part-time 4x4 XJ get away with an Aussie in the front?