After far too many hours upgrading connectors, virtually all of the wiring, the BOB, and other components trying to pinpoint the problem, I noticed a burn mark on one of the insulation displacement quick connectors supplying power to one of the driver boards.  Although the driver seemed to be switching correctly to drive the stepper when I went through a diagnostic, the size of the burn mark was an indication that there had been arcing going on which must have produced high enough voltage spikes to blow all of the driver boards.  The first new driver installed works perfectly.