I agree with Curb, seeing as I own a dog that used to have the same if not worse behaviour as you've described, I know it only get's worse until you tackle it head on with the right methods.
Socializing like "normal" people do with puppies may not be an option if it's gone too far. I know with Roxy, by the time I realized it was a problem, if I were to "socialize" her with strangers on the street she would've bit them. I went to school, have been for over a year now, and after some hard work and lots of treats she's definitely improved.
Although I've come to the conclusion that there are some dogs that will never be that happy go lucky dog that runs up to strangers with a tail wagging looking for a pat. Some dogs would just rather not, but you CAN work with them, train with them and show them how to behave politely in the presence of strangers.
My trainer calls it "the edge". Especially considering the breed, I would start looking for a reputable trainer in your area like Curb said. I took a set (8weeks of privates) to work on some basic obedience skills and to introduce Roxy to my trainer, then we went into a group.
The main method I used with Roxy for coming across strangers on our walks or for speaking to people that she didn't know/didn't like was distraction/reward. I could read Roxy's body language when she was thinking about lunging or snapping/snarling, and before she got to that stage, I put a treat right in front of her face, lured her atttention, than asked for a sit. Waited a few seconds, REWARDED heavy praise!
But there was a lot of work before we even got to that point.
With all of this being said, and not meaning to make an out of control, aggressive dog sound like a good thing, I'm awful proud to own a polite, well mannered dog that I feel safe with walking down the street at night time, even in the shadiest of areas. But before I worked with her, I couldn't control her, and it was more of a hassle to go out for walks, (not to mention seriously embarassing).
Good luck!
