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.