Aggression?

Question
We have a one year old female australian shepherd. She's very smart and I tought her several tricks which she was able to pick up very quickly. She's very affectionate and sweet. But we have a big problem with her. When we take her out she goes crazy about EVERYTHING!! People, dogs, cats, cars... As soon as she sees for example our neighbors outside, she will shoot off and jerks my arm off (we have to have her on a leish) She barks very aggressively and shows aggressive behavior towards anybody that gets in her way. Like if I try to pull her back or get in front of her to block her sight, she snapps at me and growls. It's impossible to take her for a walk in the neighborhood because she really goes nuts. It becomes harder and harder for me to handle her when I take her outside. I'm only 110lb so when she goes off I have a really hard time stopping her cause she pulls me along. I think she's just curious and maybe that's her way of socializing but I don't wanna let her go and take the chance that she tears somebody up... Also we have 2 cats and she's fine with them. She plays with them just fine, even though she sometimes gets too rough but that's only because she is so much bigger than the cats and she gets too excited. What can we do to stop her aggressive behavior everytime we take her out?

Answer
I have no idea how well you have socialized this dog in terms of introducing her to new people/places/things from a very early age at LEAST FOUR TIMES a week for extensive period (minimum one hour.)  I also have no idea how heavily you have contributed to her apparent growing aggression.  Most owners DO heavily contribute by attempting to calm the dog, using verbal reward cues inappropriately, putting hands on the dog (construed as reward), etc.  Your dog may very well be demonstrating approach/avoidance which can result in fear.  You need to have this situation observed and evaluated by an expert, it cannot be adequately addressed in a text box.  Call the veterinary college in your geographical area and ask for referral to a Ph.D. or DVM certified applied animal behaviorist.  Dogs don't "socialize" themselves...owners do it.  You've dropped a stitch somewhere or made an error in your response and I can't determine which that is from reading a text box.