Softissohot:
Sounds like a classic case of sciatica, we see this in patients of all shapes and sizes in the clinic I work at. More often than not the root of the problem is that patients are too stiff through their mid back and shoulders (sitting at a computer, driving, generally not getting movement in the spine all day) and so the lower back acts as a hinge point and gets strained, which puts pressure on the sciatic nerve. Any exercises that get gentle movement through your mid and upper back will help. Also do hamstring, glute and quad stretches at least 3 times a day, as often as you possibly can. The sciatic nerve runs down the leg through the IT band, so you can get sciatica type symptoms just if the leg/glute/hip muscles are too tight.
this is great advice - when i stopped exercising and getting fatter, i started to have back issues. my gf at that time convinced me to do some yoga stretching, and it really helped - i think softissohot accurately gets to the heart of the matter.
i am bigger now, but i know i should be doing more stretching. every time my back starts to bug me, i know i am overdue. there are some great youtube videos on yoga for big people, too.