Put grape juice in a wide pot with brown sugar, raisins, and prunes. Bring to a boil, then reduce heat to a simmer and cook until it's as thick as maple syrup, to about 1 cup, 1 1/4 to 1 1/2 hours. Add balsamic vinegar and simmer another 15 minutes.
Cool, then strain, pushing on fruit with the back of a spoon (save fruit for another use, like spooning over ice cream, if you like).
* Look for reasonably priced cask-aged balsamic vinegar, sometimes labeled "condimento." Avoid cheap balsamic vinegar, which has colorants and sugars.