This doesn't really help now, but it's a good next time thing. The exact same thing happened to me, but I got a pair of vice grips on it and it came out. I figured out that because there was oil in the bolt head, it created a kind of reverse suction thing in the allen head and the socket hadn't gone down all the way. Now I take a paper towel and get all the oil out of there to it grips well.

1988 Trooper II in the works.