1996 Ford Explorer wont shift to 2nd gear.. "overdrive off" light blinking?

My 1996 Ford Explorer Limited wont shift into 2nd gears on its own, you can feel it stays in first gear and wont shift, so you have to "manually" do it by letting off the gas pedal.. When im driving it, the o/d light starts blinking as well, on the instrument panel. Does anyone know the problem