Position racks in center and upper third of oven and preheat oven to 300°F. On bone side of ribs, at corner of rack, slip a small knife under clear membrane. Use paper towels to remove completely. (You may have to make a few attempts)
In a small bowl, combine salt, pepper and brown sugar. Season each rack evenly with rub. Wrap each rack securely in aluminum foil and place on a sheet pan (they can overlap). Bake until ribs are almost tender (carefully open a foil packet to check), 1½-2 hours.
Carefully unwrap each packet of ribs and baste with sauce. Reseal packets. Bake until ribs are fork-tender, about 45 mins -1 hour. Let cool slightly and unwrap completely.
Meanwhile, prepare outdoor grill to medium heat. Place unwrapped ribs on grill and cook, basting each side with additional sauce, about 3-4 minutes per side or until slightly charred. Remove from grill and allow ribs to rest 5 minutes before carving.