Very strange behavior in dog

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Hello, everyone:

I'm new to this site and it's such a relief to find it! I hope someone out there can help.

I got my 7-year-old lab/pit mix rescue when she was 1 1/2 years old. She was a happy-go-lucky undisciplined charmer and with training she got some manners and calmed down. We lived in Queens, NY, where there was no dog run so she and the other dogs played in a cement playground. We moved a block away from Central Park less than a year later. She had so much fun with the other dogs, playing, running around, etc. But when she was three years old she started exhibiting really strange behavior. She began balking -- resisting with all her weight (75 lbs) -- walking down certain streets. It just got worse and worse and the number of streets she refused to walk on grew. She would tremble and resist going outside the apartment at night and stopped going to the park then. It took all my strength to walk her. She was diagnosed as having severe noise anxiety, her fear being thunderstorms, gunshot sounds and garbage trucks. I was given tranquilizers for her, but they didn't work. It was recommended that I might want to try Prozac at a future date. In the meantime, she started getting aggressive to the same dogs with whom she so happily played. After she attacked a friendly golden who did not provoke her, I took her away from the rest of the dogs who ran around during offleash hours in the morning. Her aggressiveness grew. She began hating huskies, then all furry dogs, then dogs that looked like her. She went after a beagle, also unprovoked. Two years ago she went into an on-leash frenzy seeing her arch-enemy, a dog who lived in our building, who was walking by, and ended up sinking her teeth deep into my leg when I blocked her view.
Needless to say, people usually recognize us and keep their dogs away, though there's always the fear that an offleash dog will approach. It's awful when that happens.

She has had two trainers. They did not work. I put her on Prozac in October. Though we adjusted the dose a lot, it never worked and I weaned her from it this summer. I am at my wit's end and am wondering if her problems could be the result of a brain tumor or some other medical problem which the vet has not found. Has anyone out there experienced this? Does anyone have any ideas? I have no money for neurological tests, but all the time in the world for advice.

Thank you so much and I look forward to hearing!