Itchy back of neck area for yr, past month got worse with rash spread to armpits and 3 swollen nodes on back neck. what could this be?

Need for examination: It is not feasible to provide a meaningful opinion without examining you and your symptoms warrant evaluation by a physician.

See a dermatologist: Constant scratching will cause enlarged lymph nodes. You need a diagnosis and probably a prescription cortisone. Benadryl (diphenhydramine) will do nothing except sedate you: do not use except as a sleep aid.