SA and AvPD seem to be very similar, just a matter of severity and how extensive it's affect... but I thought I'd read somewhere that SA is more about situations while AvPD is more about your ability to form relationships. In that regard, I think my SA is mild (I can perform reasonably well in most situations, or tolerate them at least), but, although I can talk to people in most situations, knowing how to make friends and have anything but casual conversations is a mystery to me.