You say you've replaced the input & output speed sensors, TCU "s", but what about the RANGE Switch?

But yes you're general Idea of a shift box is correct. Toggle switch's in different positions control what gear you're in. Pretty simple when done correctly. But they are mainly used for a smoother/quicker shift & to have more control over when you're auto shifts. Lastly, is great for full line boost pressure. No dropping pressures in between shifts. Very Interesting.

But lets try to get you're trans into a good state again without using a shift box. It can't be that hard to figure this out.

Jeff.