Hi Jason,Maybe the tree was simply weak to begin with and that's why it's fallen prey to whatever is eating at it.
I'm not any kind of expert with anything, but if I had a tree like that I would burn it and I think you should burn yours. If you can replace it and you have 59 other trees that aren't affected, before the infested tree can infest them, then burn it. That would kill anything on, or in the tree. From what you said, it's never been right from the start, why take the chance with your other trees?