Dual Drive uses internal hub gears combined with a freehub body to provide a set of external gears shifted with a derailleur. This particular bike had a three speed hub.
A single unit controls both internal and external gears
The control linkage for the internal gears has been disconnected in this picture.
While I had the wheel out for truing I noticed that only one of the internal gears was engaging. I remounted the wheel to confirm this by reconnecting the shifter. Yep. It was totally air pedaling in low and medium gear ranges. This was not just an adjustment problem.
With a little Internet mining I got to a tech manual on SRAM's site that showed me how to break into the thing, but I saw nothing in the schematics that suggested an obvious flaw that would lead to this kind of gear loss. The troubleshooting guide made no reference to it. Based on some residue on the end of the hollow axle I suspected congealed grease in the "lifetime lubricated" inner recesses. I flooded the hub with light oil and left it overnight.
This morning it works.