Thanks to all for the good tips. The part that has me stumped is how to figure out which magnet/wheel is causing the problem. I did mess with the P3 diagnostic, but that measures amps on the whole system.
I give the compass test a try first, but suspect all are working until the brake assembly on one of the wheels is torqued to engage the brake shoes against the drum. Then something gets shorted...a bare wire or a bad magnet. Then it quits when the brake is released.
The only solution may be to cut the wires on one magnet at a time until the warning doesn't pop up going down the road. Would rather find an easy way to figure it out in the driveway though.
Might be seeing a disk brake conversion in my future - sure tired of the drum brake problems.