Cathytoo,
We have friends that train national champion protection dogs, police dogs, drug, etc along with training problem dogs. I asked them about this case and they said it is likely not to be a physical problem though it is always good to rule that out. They also said that before you drug the dog take the dog to a really good trainer. In thier experience, dogs that behave that way generally do so out of "Fear aggression". They also indicated that, if the owner is educated and willing to work with the dog, the success rate is usually above 90%. They said that you will need to understand the breed characteristics, pack behaviour, what environmental inputs mean to the dog and how to correct the behaviour. Generally if you understand pack dynamics and what pushes your dog to do this, you can resolve the problem without drugs.
Hmmm, this is a tough one because of the three year history of getting along...followed by this relatively recent change in behavior. Based on the history provided I'm wondering if there might be a physical cause.
Any change in the fur (or does that breed have hair?) like dryness or brittleness? If so, coupled with the changes, I'd ask for a thyroid test. We've seen elevated T3/T4 levels associated with aggression in goldens where none existed before, even though the thyroids were normal to palpation. Your vet could explain that more fully.
Good luck! Pete
Ragtag Golden Retriever Rescue
Lamoille County, Vermont