No mention of having reverse - if you do and it is strong then likely you either have a stuck throttle pressure linkage or a bad pressure transducer. (A stuck TP plug could do it but not very common).

A technician would first check the throttle pressure cable at the transmission and make sure the return spring is there and functioning.
If it isn’t and the throttle cable was stuck you could have high pressure and a late shift. Check that first since we had issues with both the cable and that spring.

Put a 100 pound gauge on the governor port and take it for a ride.
If it’s not close to vehicle speed then I would replace both the solenoid and transducer.

The tap is at the junction of the main case to the overdrive unit - on the passenger side of the main case at three o’clock.