Make sure you have the correct PS fluid, but I think all the XJ had a pretty low PS fluid standard, i.e. just about any PS fluid on the shelf would meet it, just don't remember what the spec is at the moment. I've been using ATF+4 as my XJ PS fluid, although NOT specifically recommended for retro-fit to the XJ, it is recommended for other Chrysler vehicles with higher PS fluid specs than the XJ. It runs great. NOTE: ATF+4 is a superior synthetic ATF, so using ATF+4 as PS fluid does NOT mean its OK to use any old ATF as PS fluid. In fact most vehicles recommend against using Dexron and many of the older inferior ATF's and if you do, they likely will NOT protect your PS system.
You know the best way to flush PS, right? Put the front axle on jack stands, so the wheels are off the ground, disconnect the return line to the PS reservoir and run it to a bucket, Keep the Engine off the whole time, then just turn the steering wheel lock to lock left and right, the steering box will pump all the fluid out of the system. Pour a little new fluid in, and repeat to flush out all the old fluid, reconnect the return line, fill it and turn the wheels to get it pumped through and the air out. All the air won't be out until you drive it a few miles, turn the wheel lock to lock in a parking lot.
Despite what the OM says, PS fluid does NOT last the life of the vehicle, especially a nearly 20 year old vehicle. You may have a problem, but really old, sheered down fluid might be contributing to it, at least the noise.