It's just so sudden that her behavior has changed... And she's still very sweet towards people. I'm really going to have to stay on her to make sure. I really don't want to screw up what we have in this neighborhood -- all of our dogs run loose most of the time and the neighbors don't mind having this and this person's dogs in their yards, etc... And this would be just the thing to make that no longer possible.

I'm trying to think of some dominance exercises I could try.... I was thinking about feeding the two other dogs first, and as jball suggested, putting her in the submissive position when I detect that sort of aggression happening... Any other suggestions?