Yes, you are required to drive a ground rod.

NEC 250.30(A)(3) states that a separate grounding electrode system shall be installed for detached buildings.The equipment grounding conductor (EGC) that you pulled with the phase conductors in the conduit is only for grounding the electrical system within the garage. It does not provide a path to earth for lightning or other fault currents.The ground rod provides a low-resistance path to earth for these currents and helps to protect people and equipment from electrical shock. Flag for review