You may have already done this, but I'd start with a vet. That's highly unusual for a 2-year-old dog to suddenly start acting aggressively. I would suspect a medical problem. A friend of mine has a Great Dane she raised from a pup. It attacked her one day, and she was ready to put him down. But it turns out he had a tooth infection. They treated that, and he's been lovable ever since.
Definitely start with a vet, then look to a behaviorist if the vet has no answers.
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