*** UPDATE ***
It's been over two weeks and there is not much improvement. We had to go back on the Carpophan, as he regressed just two days after we initially stopped.
He has mobility but he is not the same dog. I am really concerned now. It is so sad to go outside with him. Every single time he looks at me like aren't you going to get the ball?
My friend, who also watches Radar when we go out of town, told me that Radar collided with the pole on the back patio while playing with him. This happened around the same time we started noticing the problem, but I can't say for sure this is what caused it.
I went looking for local dog chiropractors and there are only two local that are certified by the American Veterinary Chiropractic Association. The one that is actually a DVM specializes in horses only. The other is a human chiropractor with the canine training and credentials, but also much farther away from where I live.
I will be talking to my vet about next steps this week.