1994 Ford Explorer tranmission

I have a problem with my transmissoin I can back up and but I can not go in drive or overdrive or in first gear or second gear when the explorer is cold or it running it go but I f I go to the store and get back in it wont shear to drive I can back it up but not go forward so what wrong there is fliud in the transmission and it has not been chage since I had the vehicle two years now so I dont know can you tell me the problem with it