Make the marinade: Whisk the brown sugar, vinegar, onion, garlic, 1 tablespoon salt and 1/2 teaspoon black pepper in a bowl. Put the ribs meat-side down on a cutting board. Starting at one end, slip a paring knife under the membrane that covers the back of the rack. Loosen the membrane with the knife, then pull it off. Coat the ribs on both sides with the marinade; wrap in plastic wrap and refrigerate overnight.
Soak 2 cups hickory wood chips in water, 30 to 40 minutes. Meanwhile, preheat a grill to medium low (275 degrees F) and prepare for indirect grilling: For a charcoal grill, bank the coals to one side; for a gas grill, turn off the burners on one side. Cover the grate on the cooler side of the grill with foil.
Drain the wood chips. For a charcoal grill, scatter the chips over the hot coals; for a gas grill, put the chips in a smoker box and place over direct heat. Close the lid and allow smoke to build up, about 10 minutes. Rinse the marinade off the ribs and pat dry with paper towels. Work the spice rub into both sides of each rack with your hands.
Place the ribs, meat-side up, on the foil over indirect heat. Close the lid and cook, undisturbed, until the meat shrinks back slightly to expose a bit of the bone, about 1 hour, 30 minutes.
Rotate the ribs (keep them meat-side up and over indirect heat), then close the lid and continue cooking, 2 hours to 2 hours, 30 minutes. (For a charcoal grill, add more hot coals as needed to maintain the temperature.)
Meanwhile, make the barbecue sauce: Heat the vegetable oil in a medium saucepan over medium heat. Add the garlic and cook, stirring occasionally, until golden, about 1 minute. Stir in the tomato paste and chili powder until incorporated, about 1 minute. Whisk in the vinegar, 1/4 cup water, the molasses, Worcestershire sauce, honey, mustard powder, espresso powder, 3/4 teaspoon black pepper and the cayenne. Bring to a simmer and cook, whisking occasionally, 5 minutes. Whisk in the ketchup and pineapple preserves. Bring to a gentle simmer and cook, whisking occasionally, until thickened, about 45 minutes. Whisk in the liquid smoke. Let the sauce cool to room temperature, then remove the garlic. (The sauce will keep, refrigerated, up to 2 weeks.)
Baste the ribs generously with the prepared barbecue sauce. Close the lid and continue cooking until glazed, about 20 more minutes. Transfer to a cutting board; let rest a few minutes before cutting into individual ribs. Serve with more sauce on the side.
For the marinade:
For the rub:
For the barbecue sauce: