I've pulled muscles & tendons in my lower back several times since I was 20 (31 now). A bit of siatic pain usually comes along with it as the nerve gets irritated. This summer I really messed myself up to the point I could hardly walk, and I resolved to do my best to get myself into shape to support my back. I was aready losing weight and exercising regularly, but I worked in a daily stretching and core strengthening routine.
The key to my relief has been doing ab and lower back exercises combined with stretching. I stretch my hamstrings, glutes, and quads every morning, and if my back is stiff, at night before bed. In the morning I do situps, reverse crunches, and back lifts. I take care to stretch out before workouts and after. My back hasn't felt this good in years.
To the posters in this thread, if you haven't already, try working in stretching AND core strengthening. It is amazing how much support your abs and lower back give to holding you upright. If you are like me and sit in front of a computer all day, those muscles will lose conditioning. You need to keep them stretched and exercised. You also need to combine this and keep your legs and glutes stretched as these muscles give you a good deal of support as well. If your back muscles have to do all the work, they will get strained/pulled. Combine the strengthening with stretching and weight loss, and you may find some relief. It has worked well for me for what its worth.