For seasoning rub, combine all ingredients except ribs and barbecue sauce ingredients in a small bowl; stir well. Spread over all surfaces of ribs; press with fingertips so mixture adheres to ribs. Cover; refrigerate overnight.
Prepare grill for indirect cooking. While coals are heating, prepare barbecue sauce.
For barbecue sauce, melt butter in medium saucepan over medium-high heat. Add onions; cook and stir until softened. Stir in remaining sauce ingredients. Reduce heat to medium-low; simmer 20 minutes, stirring often.
Place seasoned ribs on grid directly over drip pan. Grill, covered, 1 hour, turning occasionally. Baste ribs generously with barbecue sauce; grill 30 minutes more or until ribs are tender and browned, turning occasionally.
Bring remaining barbecue sauce to a boil over medium-high heat; boil 1 minute. Serve ribs with remaining sauce.
Preheat oven to 350*. Prepare ribs as directed in step 1. Place ribs in foil-lined roasting pan. Cover with foil; bake 1 hour. Uncover, baste with barbecue sauce. Bake, uncovered, 30 minutes more. Contine as directed in step 5.