Grinding and pain at the base of the skull (I assume you don’t mean base of the shoulders where the neck joins the shoulders) is consistant with arthrosis or wearing of the occiput-C1 and/or C1-2 joints. The Occiput-C1 joint will cause more pain with flexion-extension (“yes”) motions. The C1-2 joint will cause more pain and grinding with rotational (“no”) motions.
These are not easy to treat but I would start with gentile manipulation and consideration of facet injections. You need a really good and experienced injectionist for these injections.
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.