Check Napa's and Rock Auto yourself. Each pump listed for a 2000 XJ is described as "remanufactured". None of the parts sold through Rock Auto require returning a core.
With millions of used core floating around, few manufacturers are going to spend the money casting new bodies. Most people are priced sensitive and will buy the cheap rebuilt part.
No doubt there are some good rebuilt pumps out there but it is a crap shoot. More seem to be bad than good.
The only rebuilt steering pump I would be happy with is one rebuilt by a high-performance steering gear shop. Locally that would be Lee's Power Steering, in Sun Valley, builder of steering boxes and pumps for the racing community. It's probably cheaper to buy a new PSC pump.