In this situation it was none of the usual items. It was, believe it or not, the transmission-fluid-temperature sensor. As I looked further into this, sure enough, I found factory bulletin 00-40-010 dated August 2000.
There is nothing more frustrating for any technician and/or shop owner than to encounter a late-model vehicle experiencing problems and have no diagnostic codes to go by. One well-experienced example of this would be with pre-1996 vehicles. Many different styles of electronic transmissions would not set a code for an inhibitor switch because there were no codes for the computer to produce. So when it would malfunction, it would leave the technician clueless as to the source of the problem.