From the 2000 Service manual :
10) Gauge should read below 862 kPa (125 psi), if
above, inspect the hoses for restrictions and repair as
necessary. The initial pressure reading should be in
the range of 345-552 kPa (50-80 psi).
I noticed that my pump reservoir was pressurizing also. Unfortunately, mine started making all sorts of noise during the latest cold spell. I anticipate having to change out the pump come this Monday.
Anyway, at 125 psi or even at the 50 - 60 psi, I'm not surprised that with a sealed cap that there is quite a bit of pressure on the reservoir cap.

There is also threads on how to put a ZJ or WJ power steering pump in a XJ.  It might even be a direct swap.  I was thinking about doing this maybe the next time that I swap mine.  Supposed to get a lot more fluid and makes it easier on the pump because it was designed for the V8. 

I could not find them for some reason and don't have more time to look for it.  Hopefully you can find it if you want it.  Or maybe someone else on here knows of a link.

Love that plastic pulley on the metal hub ! Not really, it was a PITA ! The metal pulley hub is so cheap that it pulled the lip right off the pulley. And that was with using a pulley puller. The low pressure hose nib also broke off during removal. So I ended up replacing the pump, pulley, and reservoir. That's the price you pay for cheap Chinese parts. The good part was that the whole thing cost under $100. Well at least the steering doesn't groan any more.