Transmission problem, 97' F-150 4.6 Auto

I have a 97 F-150, Automatic, 2WD with a 4.6L engin. When it gets up to 30 mph it slows down like the brakes are on and makes a squeeking or humming sound. when it slows to 20mph the problem stops. It will run OK in second gear. The problem started all at once. Dose anyone know what is wrong? Please help!!!