Technically, all XJ drum brakes are self adjusting, but it is not unusual for that feature to be non-functional. I periodically manually adjust mine if I am going to tow or carry a large load. Jack up rear wheel, use a flat screwdriver on the adjuster until there is some slight drag.
Or, you can periodically replace the rusty internal components, and do the self adjusting procedure of backing up and then rapidly applying the brakes. Do this several times on a system that has non-corroded components.
Or, switch to disc brakes. But that has been covered elsewhere.