I've been battling a bad back for the past year or so but I've gotten to the point where I am now able to manage it.
In my case it was a lack of mobility in my back that was giving me grief. Basically my spine was being over compressed (much of it self inflicted at the gym) which was resulting in a lack of lubrication between vertebrae and the consequential pain. What I have learned is that with the right stretches before and after loading my back I can relieve the built up stresses and avoid over compressing my spine.
As was eluded too earlier, your vest may also be a source of your problems. I have recently switched over to a William Joseph backpack/vest combo. The difference with this is huge. Rather than putting all the weight on my shoulders (and consequently through my lower back), much of the weight is instead put directly on my hips.
Another product that I would recommend are 'core shorts'. Heres the website:
http://www.coretection.com/testimonials.htmlBasically, they are spandex like shorts that I wear instead of boxers, that give my lower back support. I wear them for everything. Hockey, weight lifting, fishing, hiking, work. Trust me, these things are worth their weight in gold.