You don't use ATF in the power steering system on an XJ only power steering fluid. Go to the parts store and look on the shelf the bottle will be labled (big suprise here) power steering fluid.
Waht they said. I used DExron in mine in an emergency, and it buzzed and groaned and took a lot of flushing to quiet it down. It's better than running dry, but only barely.
Power steering fluid is similar to automatic transmission fluid, but with a very different detergent pack. In the "good old days" power steering used automatic tranny fluid, but now they are different compounds. You can use a small amount of ATF to refill a power steering box, but you should use fluid formulated specifically for power steering units.