Osteoarthritis (OA) of the Glenohumeral Joint: The findings of moderately reduced glenohumeral joint spacing without notable osteophyte development suggest early degenerative changes, which are consistent with OA. The patient's symptoms of chronic pain and reduced range of motion also align with this diagnosis.

Adhesive Capsulitis (Frozen Shoulder): This condition is characterized by chronic pain and reduced range of motion, similar to the patient's symptoms. Although the radiographs do not show significant joint space narrowing or osteophytes, adhesive capsulitis can occur without notable radiographic findings.
Rotator Cuff Tendinopathy: Chronic pain and reduced range of motion can be associated with rotator cuff tendinopathy. While the radiographs do not show specific signs of tendinopathy, this condition is a common cause of shoulder pain and limited mobility.
Subacromial Impingement Syndrome: This condition can cause chronic pain and reduced range of motion due to mechanical impingement of the rotator cuff tendons. Although the radiographs do not show significant acromial spurs or osteophytes, subacromial impingement syndrome is a possible diagnosis.

Septic Arthritis: Although the radiographs do not show significant joint space narrowing or osteophytes, septic arthritis can occur without notable radiographic findings. This condition is a medical emergency that requires prompt diagnosis and treatment.
Glenohumeral Joint Infection: Similar to septic arthritis, a glenohumeral joint infection can occur without significant radiographic findings. This condition requires prompt diagnosis and treatment to prevent serious complications.
Tumor or Metastasis: Although rare, a tumor or metastasis can cause chronic pain and reduced range of motion. It is essential to consider this possibility, especially if the patient has a history of cancer or other risk factors.

Rheumatoid Arthritis (RA): Although RA can cause chronic pain and reduced range of motion, it is less likely in this case due to the lack of significant radiographic findings, such as joint space narrowing or osteophytes.
Avascular Necrosis (AVN) of the Humeral Head: AVN can cause chronic pain and reduced range of motion, but it is a rare condition that typically occurs in specific contexts, such as trauma or steroid use.
Glenohumeral Joint Instability: This condition can cause chronic pain and reduced range of motion, but it is less likely in this case due to the lack of significant radiographic findings, such as joint subluxation or dislocation.