1. How to differentiate staph infection in arm pit to swollen lymph nodes? 2. What does a higher WBC mean when having undiagnosable luq pain with otherwise normal X-ray and blood tests

Staph. Infections are usually in the skin, whereas axillary lymph nodes are in deeper tissues. But you can have enlarged lymph nodes from skin infections. Elevated WBC count means infection, inflammation, drug effect or, in rare cases, malignancy. Ask your doctor for more specific answers in your case.
It can be differenti. A good doctor can differentiate these two entities without much difficulty by careful physical examination as the nodes are much deeper than skin infections caused by staph which will involve primarily skin and will produce redness, local pain and heat. Your doctor should be able to sort out your other problems also....A ct scan of the luq might provide the answer.