85 foxbody tranny issues

MY girlfriend has an '85 5.0 and she is having issues with the car shifting she said that the car won't shift gears in drive she has to manually shift them. I checked the tranny fluid and it was fine. i have no clue what it is i have been told about some vacum modulator controls the shift points and that might be the problem but i don't know its an aod and i mainly work on manuals so and this stuff is new to me any help would be great thanx.