You need a second opinion. It does sound like a disk problem, and if it is, the dog should be on strict crate rest and you should be thinking about surgery. A disk problem is not diagnosed by staring at the dog, and feeling the dog's spine may help some if the vet finds a painful spot, but it isn't definitive. X-rays are not the best way to diagnose it, either. You really need advanced imaging like a myelogram or MRI. I would either ask the vet for a referral to a specialist, or at least try another vet as your vet isn't being much help.
There is a condition that affects German Shepherds more than other breeds, called degenerative myelopathy (DM). Below is some info on it. I wouldn't think of that except that you said the prednisone isn't helping. How long was she on prednisone, and was she being allowed to move around while she was on it?
I think you need to rule out a disk problem (and do that ASAP) before you consider DM. With a disk problem, time is of the essence. Surgery can help but the sooner you do surgery, the better the chance of a positive outcome. Until you know what is going on, do not let the dog scoot or drag around, keep her in a crate, x-pen, or small room except to go potty. You do not want her to move around and risk making it worse.