Tranny not holding 1st gear

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.
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The upshift speeds are indeed controlled by mechanical means, either linkage or a cable.
If the WOT upshift speeds have always been low, it might just need a simple linkage adjustment but if this condition has occured suddenly, suspect some sort of breakage on the linkage. Have someone sit in the vehicle and floor the throttle; from under the hood push rearward on the linkage, if there is more than 1/8"-1/4" of travel remaining in the linkage adjust the top rod (lengthen it).
A common failure point is the weld that connects the vertical throttle shaft to the cam plate in the transmission valve body, the broken weld prevents the linkage movement from reaching the throttle valve that controls the upshift speed. This weld can be checked by disconnecting the linkage from the top transmission lever and manually running the lever through the range of its travel. There should be distint stops at each end of the lever's travel, if there is any lost motion (slop) in the lever's travel the weld has broken.
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