Just curious- but do you have any form of scoliosis? Might be something to ask about if there are no disc or nerve problems. Were the image studies all negative for any compression fractures, stenosis, or spondylosis/spondylolisthesis?
If it's muscle pain and/or rigidity, Darvocet or any pain medication won't be a big help. Ask your doctor about Skelaxin- it's safe to take and drive and have a normal life without the nasty hangover effect that Flexeril has, plus a lot of pain clinics and standard medical offices have a ton of Skelaxin samples or a free month's worth coupon, just something to think about =) Good luck with your back, I know how frustrating it can be when you're young (I'm only 23) but you feel so broken and like you're trapped in an old body... or in my case living with a half metal body that doesn't even work right >.<