There is currently a need for compact actuators capable of producing large deflections, large forces, and broad frequency bandwidth. In all existing active materials, large force and broadband responses are obtained at small displacements and methods for transmitting very short transducer element motion to large deformations need to be developed. This paper addresses the development of a hybrid actuator which provides virtually unlimited deflections and large forces through magnetorheological (MR) flow control and rectification of the resonant mechanical vibrations produced by a magnetostrictive Terfenol-D pump. The device is a compact, self-contained unit which is capable of producing large work output. To achieve large output force, hydraulic advantage is created by implementing a driven piston diameter that is larger than the drive piston. Since the pump operates at high speeds, a fast-acting MR fluid valve is required. The paper presents a four-port MR fluid valve in which the fluid controls its own flow while carrying the full actuator load. A multi-domain model of the device was developed with the primary goal of analyzing and demonstrating the MR fluid valve concept. A research valve was designed, constructed, and tested for purposes of model parameter identification and validation, and analysis of device behavior. Deflections of over 6 in are demonstrated with the device presented here.