need transmission for '88 F150 Automatic 3-speed 4x4 350 how id?

need transmission for '88 F150 Automatic 3-speed 4x4 350 how id? I need a transmission for an 88' F150, automatic, 3 speed, 4x4, 350 motor. What do i need to find (or see when buying) in order to make sure it will go in my truck.