I have never had to do anything harder in my life then put down Leo. He was just shy of 3 years old, and the best dog you could ask for 99.9% of the time. He was born at the rescue and had been placed and returned once before he came to us. He had always been randomly aggressive, but we did our best to identify the triggers and avoid them. We talked with behaviorists and trainers and did our best, but knew we were fighting a losing battle. There was no way to identify what set him off. You could take his food away no problem, but if he was finished eating and trying to destroy the food bowl he would try to bite.
The only time he made contact biting was the day we put him down. Something just wasn't wired right, yet we still felt guilty that we were doing the wrong thing. He came to us when he was 10 months old, we only had him for 2 years. I know that the 2 years Leo spent with us were the best years of his life, he had a huge yard and a warm bed to sleep in…he was spoiled rotten. I think when you rescue a dog you feel even more responsible for them because not only did you give them a home, you saved them...you pulled them from a life of uncertainty.
My wife had a hold of Leos collar and he turned and grabbed her hand. She spent 6 hours in the emergency room and 5 months later still has problems using the little finger on her left hand. It was amazing how much damage was done so quickly. I called everybody I could think of that night. I called the basenji rescue we got him from, the trainer, the vet...and everybody said the same thing. We didn't put Leo down because it was the best thing for us, I still miss him every day. It was the the kindest thing we could do for Leo.
I know what you are going through with Tucker is extremely hard, but the years he spent with you are by far the best years of his life. I hope you are able to find someplace for him, but if you can't he won't be alone in the fields on the other side of the bridge.