Thanks for your continued interest and concern. I just talked to the vet this afternoon. He suggested either taking Chance to a surgical specialist (who would probably do a myelogram), or going ahead and scheduling a CT scan (I was wrong, there is a facility in Nashville). His preference was to go ahead and do the CT scan since it would provide the most information and would be useful if surgery were recommended.
I asked him about seeing a chiropractor, but he didn't know there was one in town and couldn't say whether it would do any good or not. Right now, my wallet is doing most of the thinking -- chiropractor, $95, CT scan, $700... I am leaning toward calling the chiropractor first to rule in or out any possible "simple" solution. We've had x-rays done, mostly to see his hips, but in both cases his spine was quite visible and all the disc spaces looked normal. There was a mild abnormality in his hips, but it hadn't changed in the year between the two. There haven't been any injuries that I can remember, but he loved chasing the horses and may have twisted something just wrong.