Greg, I'm sorry I didn't see this until now. It sounds very much like what Sugar went though. It started in Sept a year ago, she came in one evening from playing in pain and could barely walk. I won't go through all the details (unless you want to hear ) but she ended up with a course of pred, which was repeated when she had a relapse, and Tramadol for her pain. What really helped was acupuncture. She started in late October, and by the end of the year, she was completely back to normal. Now, normal for her was an arthritic 12 yr old, but she could get around, up and down the steps, and even went for short walks. Her passing in July had nothing to do with her back end problems.
Since Isis got "clean" x-rays, I would really encourage you to try acupuncture. It usually takes a few treatments to show improvement, but it works! I can give you the name and number of the vet I used, and I know Spencer's mom has someone else she uses.