1986 Ramcharger 5.9L Automatic

Truck runs fine and achieves 3rd gear at about 25 mph as the Haynes
Manual says it should. Problem is that it does this even if I have the
accelerator floored. I can't hold first gear past the normal point for
light accelerator, same goes for 2nd gear. Further, I can't get a
downshift from flooring it at cruise speed in any gear.

As I understand it from the manual, there is no vacuum involved with
this engine/tranny combo but rather it is all mechanical. I would
appreciate a confirmation on that fact. If that is the case, it would
seem that there is a significant adjustment to be made with the
throttle rod.

What shift points (in Mph, no tach on the truck) should I be looking
for when the accelerator is floored? How sensitive is the process of
adjusting the throttle rod? Can I overdo it and if so what is a
suggestion for amount of incremental adjustment until desired result is
achieved?

Any other thoughts are appreciated. Have learned much reading all of
the posts, thanks.

P.S. I do have known vacuum leaks that are occupying my time, but
thought that had nothing to do with the tranny on this truck.