In a small bowl, combine brown sugar, cumin, paprika, chili powder, onion powder, salt, and pepper. Sprinkle rub mixture on both sides of the ribs. Place each rack of ribs in a large foil baking bag, seal, and place the bags on a large baking pan. Refrigerate overnight.
Heat oven to 300°F. Place baking pan with foil bags in oven, and roast 2 hours. Remove ribs from bags, brush with barbecue sauce, and grill 10 minutes per side.