Mix together the garlic, rosemary, brown sugar, vinegar, cayenne, salt, and pepper. Rub over ribs and transfer to a zippered plastic bag. Marinate, chilled, 8-24 hours.
Preheat oven to 425 degrees F. Place ribs meat side up in a roasting pan. Pour ½ cup water in the roasting pan, and cover tightly with foil. Roast ribs until meat is very tender, about 1½ to 1¾ hours. Remove pans from oven and transfer ribs to a platter. Add 1 cup hot water to the roasting pan and scrape up brown bits. Skim off and discard fat, then transfer liquid to a medium sized skillet. Add vinegar and brown sugar and bring to a boil, stirring occasionally. Continue to boil until reduced, thick, and syrupy, about 15 minutes.
Preheat grill to a medium heat. Brush some glaze on both sides of ribs. Grill ribs, turning occasionally, until ribs are hot. Brush with more glaze and serve warm with remaining glaze on the side.