If only someone made Xj specific plug n play headlight looms with driving light wiring inluded.....
Realistically you should be able to tap into the wiring near the steering column and run it through a switch out to your relay.
If it were me and you are going to tap into the headlight wiring at the front I'd be running it through a fuse (say 5a close to the tapping point) to be on the safe side.....