Hopefully you were stopped when you were trying to engage the ARB.

As OOP's mentioned it isn't normal, and it shouldn't stop leaking when up on a stand. Check the diff housing fitting and see if it is tight.

W/it up on the stands can you get the ARB to engage?

If air is coming out the diff vent, either the diff housing fittings are installed incorrectly, or your ring gear ate through the hard line.

ARB changed the diff housing fittings a few years ago from threaded to a compression style, and a lot of installers have had issues with them. If you tighten them too much you tear the O-ring, if you don't tighten up enough, you get leaks. I had the same issue w/the ARB in my '91. After talking to ARB/USA in WA, and installing new o-rings the way the tech told me to, mine has been leak free for a few years now. Fortunately the tech also sent me the old fittings so, if I have to get into it again, I will install the old style fitting.

I would return to the shop that installed the ARB, and have them remedy the situation, unless they set up the 3rd, and you installed it.