Just wanted to give an update. It turned out that the transmission controller was working fine. It was CORRECTLY dropping voltage to the high speed solenoid to shift to high speed as designed. Apparently over the years, someone replaced the high speed solenoid in the valve body with one that was for the low/ reverse side. Low/ reverse are the same part number, but high speed is different. Solenoids look identical, but are apparently different internally. Replaced high speed solenoid and all functions are good. Thank you to Mick at Atlantic Lift Trucks out of Baltimore Md for getting me going