Adhesive Capsulitis (Frozen Shoulder): This condition is common in diabetic patients and can cause shoulder pain and stiffness. The onset of pain over the last 3 months, combined with the patient's diabetic status, makes this a highly plausible diagnosis.

Rotator Cuff Tendinopathy: Given the patient's age and the presence of mild AC joint degenerative changes, rotator cuff tendinopathy is a likely cause of shoulder pain. This condition can result from wear and tear over time.
Subacromial Bursitis: Inflammation of the subacromial bursa can cause shoulder pain and is often associated with rotator cuff tendinopathy. The patient's symptoms and the presence of degenerative changes on the X-ray support this diagnosis.
AC Joint Arthritis: Although the X-ray showed only mild degenerative changes, AC joint arthritis could still be a contributing factor to the patient's shoulder pain, especially if the patient has been using the shoulder more heavily or has experienced recent trauma.

Septic Arthritis: Although less likely, septic arthritis is a serious condition that requires prompt diagnosis and treatment. Diabetic patients may be more susceptible to infections, making this a critical diagnosis not to miss.
Neer's Descent or Rotator Cuff Tear: A significant rotator cuff tear can lead to severe pain and disability. Early diagnosis is crucial for effective management and to prevent further complications.
Referred Pain from the Cervical Spine: Cervical spine disorders can refer pain to the shoulder region. Given the potential for serious underlying conditions (e.g., cervical radiculopathy, spinal stenosis), it's essential to consider and rule out cervical spine pathology.

Polymyalgia Rheumatica (PMR): This inflammatory condition can cause shoulder pain and stiffness, among other symptoms. While less common, PMR should be considered, especially if the patient has systemic symptoms like fever, weight loss, or malaise.
Amyloid Arthropathy: In rare cases, amyloid deposits can cause joint pain and stiffness. This diagnosis would be considered if the patient has a history of conditions associated with amyloidosis or if other diagnoses have been ruled out.
Tumors (Primary or Metastatic): Although extremely rare, tumors affecting the shoulder region can cause pain. A thorough investigation, including possibly advanced imaging, would be necessary if other diagnoses are excluded and the patient's condition worsens or does not respond to treatment.