I don't recall the exact numbers, but 12in sounds reasonable. Also, it sounds like you found where your leak is and that would cause the defroster problem you mentioned.

No, your front diff is not "posi trac"/limited slip/locked/etc unless someone put an aftermarket unit in.

Have you tried just turning the hubs to LOCK manually? That should get your 4x4 working until you get a chance to fix your vacuum leak.