Jeff's procedure sounds about right.
I've never changed diff fluid on an XJ before but it's got to be easier than a Ford 9" (you have to remove the axles and pop the pumpkin). Something I do with eyes open!
For the rear diff, you might want to raise the front of the truck a little (ramps or 2x4s or an unlevel driveway) to help drainage. Then, simply remove all of the bolts holding the cover in place. More than likely the cover will be stuck in place by the gasket. CAREFULLY pry the cover loose with a putty knife or screwdriver (you don't want to bend the cover) at the bottom of the pumpkin. Have a drain pan ready to catch the old fluid. Allow a sufficient amount of time for all of the old fluid to drain out. This would be a good time to run to the auto parts store for a new diff cover gasket, new gear lubricant and some silicone gasket sealer. Use the putty knife or razor blade to remove the old gasket from the cover. Wipe any remaining fluid drips from the pumpkin and clean both with something that cuts the oil (gas or paint thinner). Apply a thin bead of gasket sealer to the cover and pumpkin. Orient the gasket on the cover and then hold both in place on the pumpkin while you replace the bolts, finger tight. Perhaps someone else can provide a bolt tightening sequence, torque rating and amount of gear oil required. The fill plug should be on the driveshaft side of the diff. I would think the front diff would be a similar procedure.
Best of Luck - Dave.