What Causes Persistent Pain On Top Of Rib And Lower Back Causing Stiff Body?

62 year-old male, retired. I have psoriatic arthritis, diagnosed some 15? years ago. Take methotrexate, 6 per week. Controlled, basically. Maybe 8? years ago I had a shingles attack on chest and shoulders I remember. Truly hateful episode. I have for some years had a constant dull/mild pain which seems located on top of a single rib on my right, at side, one of the bottom ones. If I press anywhere the skin on top of the rib, and along it for several inches, it is sensitive. My rhumatologist links the pain to some light degeneration of a d6/d7 spine joint, seen on x-ray, but I am unconvinced. I cannot provoke any pain by spine movement. The pain in this rib has been very constant, until recently when it has been increasing in intensity. Sleep, which was not easy before, now starts to be difficult. I am now wondering if I have the symptoms of a postherpetic neuralgia (PHN) case. This increased rib pain is occurring simultaneously with some lower back pain, which makes me especially stiff in the mornings. I see myself becoming more and more immobile, and it frightens me. Can you help by giving me the title/description of the specialist in these matters, that I should see, please? What kind of test could prove it to be PHN?