Chronic Lymphocytic Leukemia (CLL) with lymphadenopathy: CLL can cause lymph node enlargement, which may lead to a stiff neck if the lymph nodes in the neck are involved. This is a direct correlation between the symptoms and the disease process of CLL.

Viral or bacterial meningitis: Infections like meningitis can cause stiff neck (nuchal rigidity) and may be more common in immunocompromised patients, such as those with CLL. The immune system suppression in CLL patients makes them more susceptible to infections.
Lymphoma: Given that CLL is a type of leukemia, other lymphoproliferative disorders like lymphoma could also present with similar symptoms, including lymphadenopathy leading to a stiff neck.

Spinal epidural abscess or hematoma: These conditions are medical emergencies that can present with neck stiffness and require immediate intervention to prevent serious complications, including paralysis. Although less likely, they are critical to rule out due to their potential for severe outcomes.
Tuberculous meningitis: In immunocompromised patients, tuberculosis can reactivate and cause meningitis, which is a life-threatening condition if not promptly treated. The stiff neck could be a symptom of this condition.

Rheumatoid arthritis or other autoimmune disorders: While less common, certain autoimmune conditions can cause both lymphadenopathy and symptoms like stiff neck, although this would be an unusual presentation for CLL.
Castleman disease: A rare disorder that involves an overgrowth of cells in the body's lymph nodes, which could potentially cause lymphadenopathy and lead to a stiff neck, though it's much less likely than other causes.