Watt's the best way to run a new ground to my garage?
What you need isn't a ground rod -- it's an EGC (ground wire) run back to the rest of the grounding system, so that current can use it as a safety return to the neutral/ground bond in the main panel.

Fortunately, under the 2014 NEC -- you can run your new ground wire to a suitably sized branch circuit ground, as well as to the equipment ground conductor system, or to the main panel.  It can also follow any route you wish -- as it's a safety-drain of sorts, it doesn't have to be routed with the other circuit conductors.