1. Remove membrane from back of ribs. In a small bowl, combine dry rub ingredients and mix well. Set aside 1 tbsp of the rub for liquid seasoning mixture and apply remaining rub generously to front and back of ribs. Pat gently to ensure that rub adheres to meat.
2. Build a fire (a combination of charcoal and wood) for indirect cooking by situating the coals on only one side of the grill, leaving the other side void. When coals register 250 degrees , place slabs meat side up on grill over empty space. Close the cover, and cook with indirect heat for 21/2 hours, adding coals as needed to maintain 250 degrees .
3. Cut 2 pieces of foil large enough to wrap each slab of ribs completely and stack them to create a double layer. Remove ribs from grill. Place each slab, meat side down, on a doubled foil square.
4. In a small bowl, combine liquid seasoning ingredients, including reserved tbsp dry rub. Pour 1/2 cup of the liquid over each slab, then tightly wrap and seal foil. Place wrapped slabs back on grill, close the lid and cook over indirect heat for 1 hour.
5. Remove ribs from grill, unwrap and discard foil and liquid. Brush barbecue sauce on both sides of ribs. Return ribs to grill for 15 minutes or until sauce caramelizes. Remove ribs from grill, cut and serve with additional sauce, if desired.