A powered, directionally steerable trailer jack that can be used to elevate and lower a trailer tongue and having a caster wheel assembly affixed to a lower end of the jack. The caster wheel assembly includes a shroud having the wheels' axle journaled for rotation at a lower end thereof along with a pair of shafts also journaled for rotation in the shroud and oriented parallel to the axle. A chain and sprocket arrangement provides a low-speed, high torque transmission between one of the shafts and the axle and a high-speed, low torque transmission between the other of the shafts and the axle. The trailer jack assembly can be tilted out of its vertical disposition to provide adequate clearance when the trailer is being pulled over uneven road surfaces. The power for driving the wheels may be provided manually by means of a crank for turning one or the other of the shafts or by an electric motor coupled one of the shafts.