What can cause swollen Inguinal Lymph Nodes (also neck, arm pit and behind knee on occasion) consistently for almost 2 years? (All on left side only)

Lymphadenopathy. When lymph nodes enlarge it is often due to their performing their designated function in helping to fight off an inflammatory process. If these remain enlarged, are fixed to underlying tissues or stony hard in consistency they should be seen by a doctor. Or if you are overly concerned, do that for reassurance. Best wishes.