In a small bowl mix the rub ingredients.
If using wood chips, soak in water for at least 30 minutes (no need to soak wood chunks).
Using a dull dinner knife, slide the tip under the membrane covering the back of each rack of ribs. Lift and loosen the membrane until it breaks, and then grab a corner with a paper towel and pull it off. Season the racks all over with the rub. Let the racks stand at room temperature for 30 minutes before grilling.
Prepare the grill for indirect cooking over low heat (250˚ to 300˚F).
Meanwhile, in a medium saucepan over medium-high heat, melt the butter. Add the onion and cook until slightly softened, 4 to 5 minutes, stirring occasionally. Stir in the ketchup, tequila, vinegar, brown sugar, honey, chili powder, and granulated garlic and bring to a boil. Reduce the heat to medium-low and simmer until thickened, 18 to 20 minutes, stirring occasionally. Remove from the heat and stir in the lime zest.
Brush the cooking grates clean. Add one wood chunk to the charcoal or one handful of the wood chips to the smoker box of a gas grill, following manufacturer’s instructions, and close the lid. When the wood begins to smoke, place the racks over indirect low heat, bone side down, and close the lid. Cook until the meat has shrunk back from the bones at least ½ inch in several places and the meat tears easily when you lift each rack, 3 to 4 hours. Keep the temperature of the grill between 250° and 300°F. If you’re using a charcoal grill, replenish the charcoal as needed to maintain a steady temperature. Leave the lid off the grill for about 5 minutes to help the new briquettes light. Add one wood chunk to the charcoal every 45 minutes, or drain and add one handful of the remaining wood chips to the smoker box every 20 minutes before the old chips burn out. During the last 30 minutes of cooking time, lightly brush the racks with some of the sauce.
Remove the racks from the grill and let rest for 10 minutes. Just before serving, lightly brush the racks with sauce again. Cut the racks into individual ribs and serve warm with remaining sauce.