A trailer has a body, a primary wheel, and an adjustable leg for providing secondary support. The leg has a terminal end moveable between retracted and extended positions and a secondary wheel rotatably attached to the terminal end, such that the secondary wheel is off the ground when the terminal end is in the retracted position and is in contact with the ground when the terminal end is in the extended position. A pneumatic or hydraulic drive unit is mounted on the trailer body for adjusting the leg to move the terminal end between the extended and retracted positions. A drivetrain is mounted on the trailer body and operatively connected to the secondary wheel for driving the secondary wheel. A controller controls operation of the drive unit to automatically adjust the leg to maintain balance of the trailer, and controls operation of the drivetrain to drive the trailer.