Tara, I'm so sorry you're going through this. I know how heart wrenching it can be. My D2, Reggie, was initially diagnosed with zinc toxicosis three or four years ago and after several chelations, we just decided that it couldn't be that. He pulls new feathers out and has a shaking issue like you would see in heavy metal poisonings but all tests come back fine. I finally gave up a year or so ago and just try to love him for who he is now even though he's a very different bird.
There is a test you can run for bornavirus that they are pretty sure is the cause of PDD. If your bird is shedding bornavirus, it will come back positive, although what to do from there is always another question.