Thanks. I chose the SRAM Dual Drive manual over that because, well... it's a SRAM Dual Drive. Silly me.

I took it to the LBS. They said there was something going on inside the Dual Drive clickbox, but since the mechanic doesn't speak English and the owner's translator skills are less than optimal, I don't know any more than that. But after they did whatever they did to the clickbox, all it needed was an adjustment to the low limit screw, and it seems fine now.

And I still don't know exactly what was wrong. One of the down sides to living in a Spanish-speaking neighborhood...