Transmission modulator valve

[Larry_NM]

I have a 1995 Ford Explorer 4WD Eddie Bauer. The transmission was rebuilt in September 2018 at about 214,000 miles. It has been shifting fine until about the end of December 2019. It still shifts smoothly but now tends to shift continuously between 3rd and 4th going up a slight incline. Could this be the result of a modulator valve going bad? If you want to point me to other threads then that would be fine. Thanks for any help.

 

[96eb96]

(Guessing here..) Maybe torque converter lockup issues? Maybe misfires? Bad plug or coil?
These transmissions are electronically controlled, no modulator valve.