What is the history on the trans ? What is done to it ?
Is the modulator valve in right? Modulator connected to full vacuum? Modulator pin preload checked ? If the modulator you are sure is all in right and functioning it'd have to be a valvebody issue
History; I bought the trans nearly 20 years ago, was still in the original car it came in. I installed it in my 63 F100 with a 390 FE.
Whats done to it; As far as I know it stock,
Its been to several shops since then and as far as I know the modulator is in correct, not sure on the pin preload check. If its something I can check with my limited knowledge I will.
Some more on the problem, a couple years back the wife bought me a B&M shifter, I installed this and shifted into low with it and started to shift back up when it wouldn't move, needless to say I put so much pressure on the shifter that it destroyed the cable. It shifts out perfect when taking off in Drive.
Last year I put it up on the rack and had a buddy sit in it while I tried to shift from underneath, I put a cheater bar on the trans shifter arm, when I saw the arm starting to bend I stopped.
I know very little about transmissions, but to me it feels like that what ever it is is very solid. .
EDIT; I have two valve bodies that I have used, does not seem to have any change to the problem.