Any vets out there? No one can figure this out!?

This is actually the third post I have made about this over the last few weeks. My dog has been in severe pain for a few weeks now. It has gotten progressively worse. She has seen 2 different vets, and no one seems to know what is wrong with her. She started out by just yelping a little when she stood up from lying down. She was put on Rimadyl, and showed no improvement. I took her back to the vet and he found bilateral muscle atrophy in her lower back and at the top of her hind legs. He said it may be a spinal issue and gave her a shot of cortisone. She was fine for a few days from the shot. We then started her on a high dose of prednisone and she went downhill quick. Panting heavily, trembling, and screaming in pain any time she moved. Took her to the emergency vet, and they x-rayed every bone/organ in her body and found nothing but a slight hip dysplaysia, which they said could not be causing that much pain. The started her on a course of antibiotics to treat possible tick born illnesses (at this point I could not afford the test) and gave her tramadol as well. She was being weaned off the prednisone as we started this treatment, and appeared to improve for a few days. Then, all of a sudden she crashed again. She screamed at great length any time she tried to walk. I speculated that it may be a pinched nerve, based on the times it seemed to hurt her. Her vet said we could try it, so now she is on methacarbomal and rimadyl. She is not improving, and earlier I saw her try to stand up and she screamed constantly until she sat down. She is afraid to move, and hasn't even used her puppy pad for over a day now. She is going back to the vet tomorrow, though, at this point I am not sure what else to try? I know it has to be something treatable since she has shown extreme improvement for a few days at a time on different combinations of meds, but nothing seems to last! I can't afford an MRI, which is the only thing I can think of. I am so terribly shaken up by this and am desparate to figure out what is wrong. Has anyone out there ever seen anything like this?? Please help!!