I have a Jamis Ranger 1.0 MTB. I am 6 ft tall, and the bike is 21". Still the handle bar that I had was probably too low and was KILLING my back. After riding around 30-45 minutes, my back would start severly hurting. Due to that before I got tired by bicycling, I had to take a break.
I went to a different LBS had a brief conversation with him and showed him my bike. He gave me different handle bars (around 4" higher). Changed the fork, and replaced the thick mountain bike tires with thinner tires (dont quite know the brand, but cost 34 bucks each and takes 85 psi).
The fork that was originally there was with shocks that came with the bike. Replaced it with solid fork with no shocks.
And today I rode the bike for 2 hours, and drained the juice out of me, but my back didn't hurt at all !!
Just wanted to share this and see what people have to say about it