Interesting. I've had the same rash as you've had on my back as well. Whether it has anything to do directly with what's going on with you, your guess is as good as mine. Since I'm quite physically active, my primary care physician contributed my rash to sweat, heat, and irritation from my clothing. None the less, mine went away after I changed my diet.
Have you had a neuroradiologist interpret your scans?
Have you had a lumbar puncture?
Have you been tested for Lyme Disease?