Preheat grill to medium. Cut each rack of ribs into thirds. Place half of ribs in single layer on each foil sheet. Combine brown sugar, paprika, garlic powder and pepper. Sprinkle ribs with spice mixture; rub in to coat evenly. Wrap ribs by bringing two sides of foil sheets to center. Leaving space for heat circulation inside packet, fold foil down. Fold in one end to seal. Through open end, add 1/4 cup water. Fold foil in to seal. Repeat for second packet. Grill 45 minutes to 1 hour in covered grill. Remove steamed ribs carefully from foil and place directly on grill. Grill ribs 10 to 15 minutes in uncovered grill brushing ribs generously with barbecue sauce and turning every 5 minutes to cook evenly.