Thanks so far for the help. Here's a little more detail: Yes, it is
the kind that wraps all around the receiver and has a little mushroom
shaped thing that goes inside where the hitch ball normally does. The
lock body is about 1/4 inch steel that wraps all around (in kind of a
U shape) the part of the trailer that holds the ball. The key inserts
and turns very freely, but I can feel the locking mechanism budge or
move even a fraction.

I've been soaking it in WD 40 all day and periodically whacking it
with a hammer. I've also been trying to pry on the mushroom thingy
whenever I can. Since the lock is on a trailer and a boat is on the
trailer, I cant do much but work on it in position. I'll keep up my
technique until it either gets free or Monday comes and I relent and
call a locksmith.

Keep the ideas coming!