It is fixed!
My version of following Fred/rfs56trk's instruction above to bypass and disconnect the kickdown switch was to use one of the screws on the kickdown switch to connect the two wires that needed to remain connected together. He followed up by alerting me that a short in that switch still had access to the system, so I separated the pair from the switch. Now the overdrive works as it should.
Thank you to everyone who gave advice!
1950 GMC 100 Panel Truck, 1958 270, 3-spd Saginaw with BW OD, 3.73 12 Bolt Differential from a 1966 C10, Front Disc Brakes Bought it in pieces 9-1976, driving since 6-1978 Follow in the DITY | 
