A. Barnhard wrote:

This is our first winter here in central NJ in a new house. We had a
foot of snow over the weekend and now we're having artic weather.
During the day there is some slight thawing in the sun and everything
refreezes at night. In the corner of an exterior wall inside the
garage water is dripping down inside the wall, dripping out the bottom
behind the drywall, and pooling on the cement floor. (1) How in the
world can the source of this problem be isolated and fixed? (2) If
this is not a do-it-yourself problem (as I suspect it is not in my
case), which building trade would be best qualified to tackle the
trouble-shooting? Thanks for any ideas.

It's an ice dam, as all of the others have said. Also the problems leading to
ice dams have been mentioned.....but what to do in the mean time to stop
further damage.

Try this site.
http://www.thisoldhouse.com/toh/know.../0,16417,21160
4,00.html

BTW I had an ice dam so large one winter the weight of it ripped the gutter
along with the fascia and soffit boards right off of the house. It also
knocked out the power to our whole neighborhood because it fell on the power
line comming from the pole in the back yard.
Good Luck
Dan.