Thank you all for your help. To answer some questions and clarify, I
have an attached garage, and I ran three insulated black wires in the
trench. One wire we taped white to make a neutral. No other wires in
the trench. I now have the grounding bar I need for the subpanel.

Can anyone explain why I need to run the ground back to the main
panel, as opposed to sinking a couple of grounding rods outside near
the subpanel? These rods would be about 60-70 feet from the rods at
the main panel. Since the inspection is done I have mostly filled my
trenches, so adding another wire will be a chore, but I want to do the
right thing for safety (I guess meeting code is not as much of an
issue seeing as the original setup passed inspection...)