Code calls for a single ground basically at the meter (The first breaker box past the meter is where it goes) in the garage you shoudl have a SUB box. the ground bus in this is NOT bonded to neutral but is connected to the master ground back at the master box.. And that is how it should be.
And there is always a way to run a ground rod. even if it's solid concrete you need to drill through.
But since the proper way is to ground back to the main breaker box ground.. and that wire should already be there.. Don't worry about it.