I rode an Epic up to a couple of months ago and had nasty back problems after every ride. I have tried an Enduro and a Stumpjumper with similar results. It is more the movement on the bike that affects my back pain. I have similar issues with a tough Cross ride. The road presents no pain at all so that is now my riding of choice.
You have a FS bike so use that as a guide but try others to see if you still experience the pain. I have similar back issues to you so I suspect you will.
Let me temper that by saying, it also depends on your riding style. I did all singletrack requiring jumps and bunny hops, lots of rocks and roots etc. If you ride an MTB but on nice prepared trails, you may have no issues on a good FS bike.
Last edited by jdon; 02-17-10 at 10:21 AM.