About one hour before smoking, make the dry rub. Combine salt, sugars, black pepper, chili powder, garlic powder and cayenne in small bowl; mix well. Generously coat both sides of ribs with rub. Set aside at room temperature for 1 hour.
Meanwhile, make the sauce. Whisk together all ingredients in bowl.
Heat charcoal or gas grill for indirect cooking at 250 degrees F.
Place the ribs, meaty side up, over a drip pan and cook for 4 1/2 to 5 1/2 hours until very tender, basting with barbecue sauce every hour. Add about 12 coals to a charcoal grill every 45 minutes or so to maintain the heat.