I hate to sound too much like I'm preaching the same ol thing, but have you tried a recumbent? I love my DF mtn bike, but I never have lower back pain on my bent (and I have chronic lower back pain, if I don't exercise enough). Not trying to sell you a new bike, but if you can't get your back to work with an upright, you might at least take a look at some bents (and there's a lot more variation in bents than uprights).

Oh, and like the others said, you should probably get a professional opinion on your back as well.

Good luck.