Jack it up so you can worked on it underneath and the drain and remove the tranny pan and verify the shift linkage inside the case is moving the manual valve correctly on the valve body and not making the manual valve stuck between two gears
Have some one shift the shifter in the car and you verify that when you remove the pin from the shifter linkage inside the tranny off of the valve body linkage that the shifter linkages doesn't move at all, the pin should move freely up and out and in and out as well as drop back into that hole with no movement on the valve body linkage at all, NONE
If it does move adjust the shifter linkage from the shifter until it doesn't move the interior linkage at all when sliding it in and out of the tranny linkage