Hi and WELCOME to YT
! That's really odd there was nothing on xray...they did xray the hip and everything? And he checked the patella to make sure it wasn't luxating? Otherwise, I'd be worrying about the back - and the vet should do full range of motion movements for the entire back and neck to see if something is off there. With back problems, some dogs will lift a leg bc they're feeling radiating nerve pain.