Get rid of the back pack.

The other year, at a Bike to Work Day event, I saw a woman on a Cannondale road bike with this Axiom rack on her bike. It's specifically designed for road bikes.

If you have back issues, you really should consider getting that bag off your back. No amount of tweaking of a road bike (especially if you compete on it) will give you the position you'd need for carrying a back pack pain free.

My $0.02.