A transmission assembly for a powered vehicle, comprising: a transmission comprising first gearing comprising a plurality of gear ranges and first selectively engageable torque transmitting mechanisms; an auxiliary gearbox driven by the transmission and comprising second gearing comprising a high range gear and a low range gear and second selectively engageable torque transmitting mechanisms; a first output speed sensor coupled to the transmission and configured to detect a speed of an output of the transmission; a second output speed sensor coupled to the auxiliary gearbox and configured to detect a speed of an output of the auxiliary gearbox; and a control unit coupled to the transmission, the control unit having memory, processor circuitry, and executable routines stored in memory to: receive electronic signals from the first and second output speed sensors, determine whether the auxiliary gearbox needs to be shifted based on the electronic signals from the second output speed sensor and on a throttle signal received from an electronic control unit mounted to a drive unit of the vehicle; and send a shift command to the auxiliary gearbox to shift to one of the high or low gear ranges while the vehicle is in operation in response to a determination that the auxiliary gearbox needs to be shifted, the control unit further comprising a shift schedule stored in the memory, wherein the shift schedule indicates: shift commands for the transmission; and shift commands for the auxiliary gearbox, and wherein the shift schedule indicates the one of the high or low gear range for a combination of the electronic signals from the second output speed sensor and the throttle signal. a transmission comprising first gearing comprising a plurality of gear ranges and first selectively engageable torque transmitting mechanisms; an auxiliary gearbox driven by the transmission and comprising second gearing comprising a high range gear and a low range gear and second selectively engageable torque transmitting mechanisms; a first output speed sensor coupled to the transmission and configured to detect a speed of an output of the transmission; a second output speed sensor coupled to the auxiliary gearbox and configured to detect a speed of an output of the auxiliary gearbox; and a control unit coupled to the transmission, the control unit having memory, processor circuitry, and executable routines stored in memory to: receive electronic signals from the first and second output speed sensors, determine whether the auxiliary gearbox needs to be shifted based on the electronic signals from the second output speed sensor and on a throttle signal received from an electronic control unit mounted to a drive unit of the vehicle; and send a shift command to the auxiliary gearbox to shift to one of the high or low gear ranges while the vehicle is in operation in response to a determination that the auxiliary gearbox needs to be shifted, receive electronic signals from the first and second output speed sensors, determine whether the auxiliary gearbox needs to be shifted based on the electronic signals from the second output speed sensor and on a throttle signal received from an electronic control unit mounted to a drive unit of the vehicle; and send a shift command to the auxiliary gearbox to shift to one of the high or low gear ranges while the vehicle is in operation in response to a determination that the auxiliary gearbox needs to be shifted, the control unit further comprising a shift schedule stored in the memory, wherein the shift schedule indicates: shift commands for the transmission; and shift commands for the auxiliary gearbox, and shift commands for the transmission; and shift commands for the auxiliary gearbox, and wherein the shift schedule indicates the one of the high or low gear range for a combination of the electronic signals from the second output speed sensor and the throttle signal.