Lower Back Tightness

I have fought this for a long time. I have always attributed it to hamstring tightness. When it flares up, a week or so of additional hamstring stretching as fixed it. Until now.

I, like everybody else, have had a lot of snow this winter. No problem; shoveling is good GPP. But 20 minutes in I have to quit. No amount of rest and stretching has fixed this. Any suggestions?