What Causes Itchy Rash All Over The Back?

I m a 74 year old mail not over weight and in good shape. I have CLL and diabetes. They diabetes is controlled pretty well. I have a reddish rash on my back from top to bottom that itches and feels like a 2nd degree sunburn. I m had it for 2 years. My doctor, my dermatologist and my oncologist do not know what is causing it. When I lean back on my car set while driving it is very uncomfortable. Is very bad and not so bad during the day. Worst at the end of the day. Not bad when I wake up in the am. I have put a steroid cream on it but it made it worse. I had a biopsy done and it did not show anything unusual. I use ceraVe itch relief before i go to bed and it does help a bit. I think it is some kind of sensory nerve issue? Any help would be great! Keith Adams.