When I press the screen wash lever the water only trickles from the jets. I suspect the pump is faulty and needs replaced...but I cannot locate it

What year is your xj? If its 97-01 it's inside the drivers side fender just above the wheel well and it's a pia to get to

It's a late 97 one of the "new" model XJ. If I can locate the pipes I'll just fit a pump from another car. I had to do that to my last Jaguar...also an XJ oddly enough, an XJ6 !.

It should be on the fluid reservoir.
I believe in a 97 it is inside the fender, you have to pull the wheel and inner fender liner to get to it.

Yeah this is a better description. But like I said its a pia, stupid location choice on jeeps part