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Even if the kickdown valve was stuck open, the trans would still shift once you got up enough speed. (providing the rest of the valve body works properly)
There are 5 valves in an automatic that make it work. Pressure regulator, manual valve, shift valve, kickdown and governor. The last 3 are the ones that make it automatic. The governor directs pressure to one side of the shift valve, when it is greater than line pressure, it causes a shift. The kickdown valve adds pressure, in relation to throttle position, to the other side of the shift valve. The 2 valves are what regulate shifts.
If you have the 1-2 shift, the valves seem to be in working order. The 2-3 shift valve could be stuck. Heat and dirt can easily cause this problem. So you can see, that once you get up enough road speed, the governor pressure would over take the line pressure and make the trans upshift. (the book should tell road speed for shifts too)
I'm not familiar with this tran, but they are all the same. You have clutches and bands that drive, or hold planet gears. Generally there are bench tests where you can blow air (instead of fluid) into an apply circuit for each of the components. You will see the bands apply and hear a thud on clutches. The valve body needs to be removed for these tests.
They use some of the components twice to compact the transmission. Such as a low/reverse clutch. That would mean it is energized in both manual valve selections. The forward clutch only works in forward, and should not be suspect. (that is the reason for the crappy belleville return spring). Knowing which components are turned on and off will help to diagnose this problem. The book will have suggestions on what part to look at. Hope this helps understand automatics, they are really easier to work on than a manual trans.
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