Sounds like it's time to pull the tranny and torque converter and bring it to the shop for a rebuild. Something in there seems to be broke, and it's very hard to figure it out unless it's apart. Sometimes it's just a piston seal that's gets blown and the tranny can't build pressure. On the other hand it could be as simple as a spring in the valve body, such as a pressure regulator spring.
To remove the torque converter you have to pull the tranny anyway. Usually if the converter is broke it will have some pieces parts scattered throughout the tranny.
Other things to check, it could be as simple as your shifter cable coming loose.
Not sure of your tranny but might be an M-40, though I am not too familiar with the more recent trannies. I'm still back in the turbo 350 and 400 days.