Synchronizer mechanisms play an important role in the selection and engagement of gears in manual, automated manual, and dual clutch transmissions (DCTs). These mechanisms rely heavily on the balancing of torque loads in cone clutches, dog gears, and from losses in the gearbox to ensure repeatable and reliable actuation, with excessive wear on friction and contact surfaces, leading to degradation of actuation and potential mechanism failure. DCTs, in particular, provide a unique operating environment for synchronizers, most notably is its actuation with the engine still driving the wheels during normal driving conditions. Thus, the consideration of increased transmitted vibrations through the powertrain must be evaluated to study the impact of these vibrations on the synchronizer. To conduct this investigation, this paper develops a detailed multibody dynamic model of a typical automotive powertrain equipped with a DCT. This includes engine models with torque harmonics that capture the instantaneous torque variations from piston firing in the engine. As the main consideration of this paper is the influence of engine harmonics, the semidefinite powertrain model is simplified to a fixed-free system and the response of the synchronizer mechanism to harmonic torque inputs is analyzed. Parametric analysis of the system is conducted to analyze the influence of variables—including gear ratio, torsional damper, system damping, and engine configuration—on the dynamic response of the mechanism. Results demonstrate the influence of each of these variables on synchronizer dynamics in the steady state, with stiffness of torsional damper having the strongest influence on forced vibration. Additionally, results vary significantly between single and dual lay-shaft transmissions.