I have never seen one installed on a KJ. The KK and KJ front crossmembers are exactly the same under the oem bumper/grille. What I don’t know is what height the steel bumper will end up being relative to your fenders, turn signals, and grille. I don’t know how much trimming you would have to do. If you do go for it, you’d be blazing the trail for other KJ owners who want a great quality steel bumper but can’t get one from ARB anymore. And ATH makes a great product and you’d be supporting a USA small business.
If I were you I would scour the internet for KJ and KK photos with the front bumper and grille off, compare them to each other, and compare to the photos on ATH’s website. You might be able to figure out whether it’s reasonable with your eye-chrometer.