I had a similar situation with Sunny a while back. Our vet didn't really know what was causing his pain/limping, but thought it was in his shoulder. We tried rest, painkillers and muscle-relaxants, but although this did improve the symptoms at the time, as soon as he returned to normal activity the symptoms would return. Eventually we took him to see a very experienced greyhound vet, who found that the problem was a slight misalignment of one of the bones at the base of his neck between the shoulder blades. He told me that this injury is very common in greyhounds, often after a fall, and would not usually be visible in an xray. After a few sessions with a chiropractor, Sunny was virtually back to normal, although he does still need occasional treatments. IMO it would be worth getting a second opinion from an experienced greyhound vet and/or a chiropractor. Not everyone agrees about chiropractic, and I was quite apprehensive myself before I took Sunny, but I have to say that I found the chiropractor very knowledgeable and the treatments definitely helped Sunny.