I hate vehicles.
Are you starting out in second gear? Almost sounds like it’s in limp-home mode, like there is an electrical fault with your transmission and it’s stuck in second gear as a safety. Either that or engine misfire/clogged catalysts, engines have a lot of ways to run poorly, but transmissions generally don’t have a reason to drag the vehicle down and still shift properly. Can you feel it upshift multiple times to 50mph?
You’re a smart cookie. It is in limp home mode, according to the transmission folks, caused by some sort of electrical malfunction.
The truck is a Frankenstein, and a lot of stuff that was mixed and matched caused electrical issues. It has an engine in it that isn’t the original make or model.
I never took it to 50mph, only 50km/h, as I didn’t want to highway drive it. It didn’t feel like it shifted once between 0 and 50km/h.