Condensation is forming on a wall of my attached garage in a very specific pattern following an a/c duct. The garage itself has no a/c, and the wall in question adjoins the interior of the house. There is no condensation in the interior space. Preliminary investigations on my part (ie: cutting out a 2 inch square in the drywall of the affected area) have revealed that there is no insulation between the duct and the drywall of the unconditioned space - in fact they touch each other. What do I need to do?

Only way to do it right is to tear out drywall and insulate the ductwork
It will cost you much more later

What is the exact size of the duct?

Moisture accumulation (condensation) is a function of cool meeting warm. Either insulate to stop migration of cool from inside the duct or "prevent the warm, moist air from getting to the sheet rock ? Sounds silly I guess but it's exactly how they keep metal buildings from sweating on the inside.