The caliper bolt heads are one thing that change frequently during a production run, even within a model year.
Not just for Jeep, but for GM and other mfrs. I just played that game with my Grand Am.
And, you never know what the previous owner or mechanic might have used as a replacement: Allen, torx, 12 point, whatever.
Also, the '84 was a bit of an orphan, what with the CV joints and all. Stuff that works at the front axle ends for the '84 won't necessarily work for any other XJ.