Left the battery disconnected overnight. Still have the issue. First start after reflash the ABS light was on. Then it went out while the car was warming up. Started driving and the code popped up when I made it to ~ 25mph, like it has been doing.
I can hear a slight speed-dependent clicking while driving slowly, coming from the RF. Maybe that's a clue. Next step is to get her up on the rack for a closer look.