I've bought aluminum skids from steel armadillo, but I don't see an XJ on their site. They have quality stuff (I've bought quite a bit), and it's a shop, not a store, so just because it isn't on the website doesn't mean anything. Send him an email or call him up and ask if you makes what you want. FMJ is another custom fab shop that I'm working with to make a custom rear winch mount bumper. They were very happy to customize, and aluminum was one of my options.
The steel armadillo skids are nicely made, and thick, but aluminum is aluminum and steel is steel. Steel is much harder to gouge, and moves a little before it tears. Aluminum is stiffer and softer, so it's easier to tear a chunk out with a rock. My first few offroading trips, which didn't include any rock crawling, already put some badges of honor on the aluminum. I was going for light weight, and want to see how these hold up long term. I'm under budget on my build, so if I rip up the aluminum and switch to steel next year, that's okay. If they last for years, I would probably just replace them with aluminum again.
If you're dragging over rocks on purpose, you might want to go with the lower cost steel which will take more hits. The general engineering rule of thumb, subject to hardness and composition, is that the same strength in steel is twice the wieght, but 1/3 the thickness of aluminum. Again, aluminum varies greatly, just like steel, but the ratio is useful as a first comparison (before looking up numbers on the steel or aluminum alloy you are choosing).
Fatigue strength, and work hardening are much better in steel, meaning if you're going to bend something and bend it back, don't use aluminum. When aluminum bends, leave it bent, or plan on it cracking. If you have to bend it out of your way, bend it as little as possible and heat it if you can. Aluminum is very subject to stress concentrations, like sharp corners or cracks. Crack propagation in aluminum is brutal compared to steel. If you see a crack in aluminum, at least drill a smooth hole just past where you can see the crack end (with a bright light and magnifying glass), but you should replace it if failure would be costly.
Steel is easier to repair pieces of it, and even an amateur weld can be very effective. Amateur aluminum repairs can be very dangerous.
So, if cost is less important than weight, and you don't need a long life of beatings out of this part, then aluminum is a great way to go. If the skid plates are for "just in case" or the hits are softer materials like mud and sand, then aluminum is even more attractive.
Try steel armadillo or FMJ offroad if you want something made. In fact, the custom winch mount bumper that FMJ is designing and building for me was about what a catalog bumper costs without the winch mount.