I agree with you that this is probably a case of prurigo nodularis which is a relentless itchy condition. Looked like you have search for systemic causes and was quite unrewarding. I find oral erythromycin quite useful in controlling the terrible itch and has to be maintained for some time. Another mode of therapy which may be of help is intralesional triamcinolone injection which provides relief to the patient and hopefully break the itch scratch cycle. Perhaps you may want to try either of these: topical capsacain, oral naltrexone or one of the newer agents like odesetron. Keep the nails short too!