Transmission started to go from first to third when start driving, what steps do I need to change ECU?

Transmission started to go from first to third when start driving, shifts at 20 MPH when let off accelerator. It will go through and into all gears fine. Just not upshift when on takeoff into second. Everything else is fine. It will downshift on hill into second. Or if accelerate to hard second will Rev out high rpm. But still pulls on high RPMs. The Transmission has been rebuilt after this happen, same results, Transmission was then replaced with another that was a good trans, with same results. So, it's not in transmission issue, I don't have the fancy computer item needed to diagnose so asking, on the truck listed above I would like to change ECU. I believe that's where I am at. What steps do I need to take?

More info, even changed transmission with a third one, know is good same results. So, three transmissions with same results. Wires good, the instrument cluster gear selection light is not lighting on all selections when choosing specifically. Has new shift cable and neutral safety replaced, just goes from first to 20 MPH or higher until let off acceleration then it shifts to third and all the rest. When kickdown by peddle will Rev high but finds second, will pull up in higher RPMs though. It Will down shift into second on hill and starts the whole series of what I just stated again to get it going. Changed transmission three times same results, so what steps do I need to change ECU?