Musculoskeletal Strain: The patient's history of lifting weights and doing household work, along with the description of the pain as feeling muscular, suggests a musculoskeletal strain. The pain's location at the back of the neck and its exacerbation with swallowing (especially thin liquids) could be related to the strain affecting the muscles involved in swallowing or the cervical spine. The mild efficacy of Advil and the presence of mild degenerative disc disease (DDD) at C5-C6 also support this diagnosis.

Cervical Spondylosis: Given the patient's age and the presence of mild DDD at C5-C6, cervical spondylosis could be contributing to his neck pain. This condition involves wear and tear on the discs and joints in the neck, which can cause pain and stiffness.
Lymphadenitis or Residual Lymph Node Inflammation: The patient's history of swollen lymph nodes in the submandibular area about a month ago could be relevant, especially if the current pain is related to residual inflammation or a new episode of lymphadenitis.
Esophageal Spasm or Dysphagia: The pain when swallowing, particularly with thin liquids, might suggest an esophageal issue such as esophageal spasm or dysphagia, although the absence of sharp pain and the description of the pain as feeling muscular makes this less likely.

Meningitis: Although the patient does not have fever, headache, or visual changes, meningitis is a potentially life-threatening condition that must be considered, especially if there's any suspicion of infection or inflammation affecting the meninges.
Epidural Abscess or Osteomyelitis: These are serious infections that can cause neck pain and stiffness. The elevated CRP suggests an inflammatory process, and while the patient's presentation is not typical for these conditions, they must be considered due to their potential severity.
Cancer (e.g., Lymphoma, Metastatic Disease): Although less likely given the acute onset and the patient's age, cancer can cause lymphadenopathy and neck pain. The recent history of swollen lymph nodes warrants consideration of this possibility.

Stylohyoid Syndrome: This is a rare condition involving the stylohyoid ligament and can cause pain in the neck and throat, especially with swallowing.
Cervical Dystonia: A neurological disorder that can cause involuntary muscle contractions in the neck, leading to abnormal postures and pain.
Torticollis: A condition characterized by a twisted neck, which can be caused by various factors including musculoskeletal issues, but also more serious conditions like infections or neurological disorders.