joe- not sure how old you are, but that may have something to do w. it, but MOSTLY i'd say you're carrying around a lot of weight and the impact on your frame may be an issue as well. major props for working hard and pushing yourself. i've also had issues w. running, keeping the weight off and having lower back problems (and found they're all related).
are you doing any weight resistance training? nothing burns weight faster and more efficiently than increasing muscle mass (muscles are calorie burning machines that work 24/7). i lost 25 lbs (from 200 to 175) when I got back on the weight bench after a 10 yr hiatus. nothing else changed. just my chest/arms and my weight.
you may want to back off on the number of miles and focus more on strength-building runs/walks (up hills, up stairs). keeping those hamstrings stretched out is critical (as other posters have mentioned).
also - if yer gonna run, i've had huge success w. a type of shoe called the Z-Coil. it's kinda goofy looking but it really helps when i run or go for walks. they both absorb a lot of punishment that would otherwise go to your frame AND change your overall posture. you can find out more here:
http://www.zcoil.com/products_styles.html
they're great if you stand a lot too. my girlfriend is a teacher (on her feet all day on concrete) and they've helped her a lot.