I had an 87 that did that. I would put it in 4 hi to drive on ice, snow etc. and as long as it was under load, it would hold. As soon as I let off the gas the slightest bit, it would pop out. At first, I didn't notice except that the sammy slipped alot. When I suspected it happening, I noticed the 4wd light go out. I would hold it in gear while I drove but that was a pain. I thought it was because the trasfer case had something worn in it. Now I'm not so sure.. The other thing I thought it might have been was the bushing or something on top not doing it's job. No way of knowing for sure now. I traded it off so I will see if the new one does it.
I think I would call petroworks and talk to the tech guy. Maybe they'll replace it or atleast tell you what could cause it. Unless you were doing some hardcore stuff, I wouldn't expect this to happen. This will make me think twice about buying one of their transfer cases if this is going to happen. How they handle this can decide whether or not any of us buy their product. I suspect they'll want to help make it right.
Let us know what they have to say or how they handle it.
-Adam