How old is William? Fletcher has started the intermittent limping/holding up paw over the last few months. My vet did xrays, twice, to get the shoulder in. No "C" (Whew), no corns, no spinal or neck problems. We tested for TBDs and Valley Fever (both neg). There were some slight indications of arthritis in the shoulder and elbow, so I started him on glucosamine and Metacam as needed. That seems to have taken care of it, except he will still hold up his paw for a little while after tear-assing around the yard . Fletcher turned 7 in March.
Wouldn't hurt to test for Valley Fever, and be sure the whole neck/spine thing is checked out. Good Luck!
edited to add, I see you are in New York, so don't bother with the Valley Fever test unless William was ever in the Southwest or CA