What others have said- is your back problem mechanical (bone) or muscular?
Have you tried acupuncture or chiropractic therapy?
Can you sit in a chair?
Can you sit in a chair with your feet elevated in a riding position?
Would a support abdominal belt help?
Can you sit and rotate your trunk 180 degrees?
I believe either renting a bike or buying a low price one until you're sure so you don't repeat your prior experience with a HD.
http://www.innova-pain.com/2012/03/0...iding-posture/
https://www.reddit.com/r/Harley/comm...f_riders_have/