I am not sure if putting in a separate ground rod is up to code. I
suspect it is not, but others can tell you better.

Since you say the feed comes underground, I assume that this is a
detached garage, entirely separate from the main house. And when you
say "12/2 romex", I assume you mean direct burial UF cable, typically
gray, with heavy rubber insulation, right?

You probably have a short somewhere in your garage, not in the feed
itself. I would shut off the power to the circuit at the main panel,
then poke around with a multimeter. You should be able to see
continuity from the hot or neutral to the AL siding in your junction
box. From there, start disconnecting wires and try to pin down which
leg of the circuit has the short.

Could be someone put a nail through the siding and into the romex in
the walls. Or you got some wear or nics where a wire goes through the
siding on the way to an exterior light, or something.

-Kevin