First, thanks for all the help and advice from all of you.

Second, I was finally able to spend some time on this problem
yesterday. I measured and disconnected this and that wire just like
the advice above. I also switched the circuit to other breakers, etc.
I'm certain that the problem lies in the wire that is buried (the very
result I dreaded). I even ran a parallel wire from the breaker box to
the shed above ground to see what would happen (~50') and the breaker
never tripped.

What I didn't try, yet, was the advice about measuring the amperage on
the circuit with a meter. I'm going to do that, still, because now I'm
interested. I'll let you all know.

Again, thanks for the help!

Greg

PS - now that I'm facing the prospect of digging (again), has anyone
ever found a less torturous method of removing sod WITHOUT a sod
kicker. I think I'd rather pull out my fingernails next time than use
one.