Hello, I hope that your dog is fine now. I'm not an expert but my neighbours dog German Shepard -Huskie mix went from being a beautiful healthy 2 and a half yrs old to having no movement in it's back legs and looking very old very quickly. It turned out to be a virus in the spinal cord. She would just stand there swaying it was heart breaking. The vet they had could not identify it either, she was even taken to a specialist in London,Ont and they live in Toronto. Apparently it was the same virus that Aids patients get and also the same Virus that appears as a growth on the legs of cocker spaniels. It is very rare.
Best of luck,let me know