I adopted Abby from the shelter about 2 years ago, maybe a little more. She was great. Stayed with me, didn't need the leash, walked at a heel, etc. About 9 mos or so ago she started getting real agressive towards other dogs when we'd go out for potty/walk. She barks and lungs for them like they are scooby snacks. She is a big dog, 70+lbs., and today it was so bad I fell and she drug me. I ended up dropping Alphy's leash and oh brother. The lady was like how about if we just let them sniff and came closer which really set Abby off, which resulted with my face in the mud. The lady backed off and said maybe not. She wasn't too scared because she stayed and talked to me and said it's uncommon for goldens to be agressive and her tail is wagging and stuff. She offered to walk with me in hopes of getting Abby used to her dog.

It's not so much I think she wants to harm anyone, her tail is wagging and she's pretty panty and stuff when she's not barking.

I think it's more of a protection thing then an agressive issue, but I'm at a loss on how to correct the behavior. My folks came from out of town with their dogs and she didn't even flinch, my friend brings his chocolate lab over and nothing (well of course, she's used to the lab, but...).
Any ideas? I'm going to look at a class, but I really want to avoid Petsmart ???