I went through the exact same thing. Back, shoulders and armpits. Sorry to say we never figured it out.
I'm thinking immune problems for my dog. Don't bother with z/d.
Try a single source food that is different from all the rest. Go makes a duck and quinoa? Ziwipeak makes pure buffalo or venison? Lot's of food avenues, try for 12 weeks, no treats. Also consider enviromental allergies? Keep a log cause it's alot to remember. Another thing I would consider is a holistic vet over a dermatologist vet. It costs us 1500.00 dollars to play a guessing game. Another thing, does her skin smell? That's another condition. She also may develop skin infections because of the scratching.