I use ATF in all my power steering systems, never had a problem. The power steering pump used on the XJ is also used on other vehicles, some of which spec ATF, either one is fine.

 

My Jeep has ATF in the PS box, came that way when I bought it. Since they are both hydraulic fluids I would say use what is in your car presently (or what the manual says after you have thoroughly flushed the system). If it aint broke and leaking don't fix it. However if it is broke take the time to fix it right, and don't mix the two fluids as they may be incompatible with each other in large amounts. BTW my Jeep leaks from the steering box which probably is the factory one so I should probably get around to fixing it sometime.....