[MattL]

Guessing here - if the hydraulic valves that control things are sitting for a while, they can freeze. Since it seems to function with everything but drive (that is likely on one valve body) I'd do the following without a schematic: Follow the hydraulic lines from the drive motors to their solenoid valve bodies and check for voltage when "operating" the drive function. If there's no voltage, it's likely an electrical problem. If there is voltage, it could be a stuck valve. (The electric solenoid valves can stick when not operated often, as is commonly recommended here to cycle all electric hydraulic valves everytime a tractor is used to keep them "unstuck" from lack of use.) Also guessing again, the valves for all the functions are likely on the same manifold, when operating other functions - do you hear an audible click on the valves? Do the drive valve(s) make the same clicking? (Assuming voltage confirmed at the solenoid.)

Take a few pictures of the valve body.