I'm in the same boat. When I got the 1009 the wipers only worked on the HI setting. I found this out far from home in a spring rainstorm. Next few months they still only worked on HI, then last month all the sudden I have the LOW setting back. The only difference than when it was stuck on Hi is that they stop whenever I hit the off switch same as you. There are a few threads on this that I've read before and I seem to remember the main conclusion was replacing the regulator. I believe that's what it was called. Also, remember General Motors likes to put as many controls as possible on the handle. I just see that as making it harder to pinpoint what's wrong. Perhaps yours will correct itself just by use, I don't know.
Good luck!
-Whitey