91 explorer trans trouble

hey all I need your help!! I have a 1991 with a a4ld transmission, heres the prob I was at the bank put the car in park when I tried to put it back in drive it just sat there would not go into gear at all reverse was the only thing that worked!! If I rev the engine with the shifter in drive it will go into gear but I must do this every time I put it into drive. WHAT SHOULD I DO NOW?? Please help. :banghead:

Check your oil transmission when the engine is running, sound like your car is low on fluid. Replace the fluid as soon as posible, cause transmission could cost you arm and legs.

already checked the fluid and it looks good. So I thought it must be something else.