I have mononucleosis and developed a full body rash, not from drugs. How can I help ease the itch as I can barely sleep. I've tried nothing but rest. I had mono for 3 days total, and now only feel the effects of the rash.

Itching. Benadryl (diphenhydramine) 50mg twice a day would help itching and insomnia, also caladryl lotion on the itching site is also helpful.
If. If you have mononucleosus and sore throat and rash please see your physician to make sure you don't also have strep throat. They often happen at the same time. In general though, itchy rashes like you describe can feel better with cool (not hot) baths or showers and taking over the counter benadryl (diphenhydramine). Avoid wearing itchy sweaters too.