A completion system and method adapted for use in wells having long lateral boreholes includes a mast assembly, a pipe handling mechanism, a pipe arm, a pipe tub, a pump/pit combination skid, a rig carrier, and a control system. The mast assembly can extend in a generally vertical direction, e.g., perpendicular to the rig carrier and/or the earth's surface, such that the rig can be placed directly adjacent to a wellbore, and the top drive or a similar drive system can be engaged with and move along the mast while remaining above the wellbore. The top drive can be engaged with a support fixture that resists torque and rotation during operation. The support fixture can in turn be engaged with the mast to transfer torque forces to the mast while being vertically movable along the mast to enable positioning of the top drive.