Preheat oven to 325 degrees. Place ham on a rack in a shallow roasting pan. Using a sharp knife, score surface of ham with ¼-inch deep cuts in a diamond pattern. Cover and bake 1¾ to 2¼ hours or until a thermometer reads 130 degrees.
Meanwhile, in a large saucepan, combine grapefruit and orange juices. Bring to a boil; cook 6–8 minutes or until liquid is reduced by half. Stir in remaining ingredients; return to a boil. Reduce heat; simmer, uncovered, 12–15 minutes or until glaze is thickened.
Remove ham from oven. Brush on ⅓ cup glaze. Bake ham, uncovered, 15–20 minutes longer or until thermometer reads 140 degrees, basting occasionally with remaining glaze.