Mystery conditions cost mega-bucks, don't they? I don't know if this will help, but it may be worth considering: when my Irish setter was young he suddenly couldn't walk one day or, if he tried (he's a real sook), he'd just limp pathetically. At first they thought it was a cruciate ligament injury, then they suggested bone cancer, but X-rays disproved these. He lost a lot of weight and condition and was put on cartrophen, which is used for arthritis in old dogs. He went on other stuff, too, which probably included antibiotics and cortisone.
Anyway, it all kind of eventually resolved by itself, slowly ... but it remained a mystery. Recently I've been reading about hypothyroid conditions, the most common type being auto-immune related where the dog's immune system destoys its own thyroid gland. This usually happens in dogs about three or so, but doesn't necessarily have any effect until the animal is older. Interestingly enough, the point was made that when this first happens, however, the dog can present with lameness and how this is not usually considered by the vet and the thyroid condition goes undetected. Good luck!