1996 Mercury Sable Very important question

Hi I have a 1996 Mercury Sable Ls. When I put it in Drive, all it does when I press the gas is rev up like it's in park. My car has gears 1, 2 and D. If I put it in 2 it drives just fine, and then I shift it to D and d works fine all the way until I stop, then I have to put it back in 2 to get it to go then. I'm really worried please help. My transmission fluid says it's full, could it be the screen?