And during all that time of no Forward, would it engage for Reverse?
There is not a switch for the inching operation, it is a manual valve that is cable operated by the left pedal.
It is not uncommon for a Toyota inching cable to become rusty, or even frayed, which can cause the cable to keep the inching valve enabled after the pedal itself returns to rest.
Take the floorboard out. With the engine not running, operate the left pedal hard and watch the movement of the inching cable bell crank arm in the top of the trans control valve. Familiarize yourself with which way it moves when the pedal is being pushed down. Now, with the pedal up........see if you can move the inching valve bell crank arm further towards its resting position. If you can move the arm any further by hand than where it stopped when releasing the left pedal, the inching cable is frayed or badly rusted.
Does this truck have a seat switch feature? Early 7Fs did not but late 7Fs do.