Although I'm big on DIY dogtraining (specifically clicker-training), I think aggression like you describe is too big a problem to handle alone and I think it's too big to treat a la Cesar Milan.
I would contact a trainer or behaviorist ASAP. It would probably be best if you could have some one-on-one sessions to begin with. I think that most trainers would say that your dog needs more attention than they can offer in a group setting. But, you can ask...
In the meantime, I would take an approach similiar to what JaspersMom described for you. That is, strangers = non-threatening treat machines.
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