If it goes away when the brakes are depressed (even slightly), it sounds like caliper pad squeal is the issue. They make a goop that can be applied between the pad and the caliper to lessen/eliminate the sound.
However, it's odd for such a noise to start up unless brake work was done recently. Is that the case?
I don't own an XJ any longer, but I still think they are the best of all the Jeep products ever made. My XJ was my favorite vehicle in my 50+ years of driving.
So...I stick around and give advice.