Pat the ribs dry with paper towels. Remove the membrane from the back of the ribs if needed, then cut the rack into three or four sections.
In a small bowl, combine the brown sugar, garlic powder, smoked paprika, onion powder, salt, black pepper, and chili powder.
Sprinkle the seasoning evenly over both sides of the ribs and gently rub it into the meat.
In a medium bowl, stir together one cup of barbecue sauce, apple cider vinegar, and Worcestershire sauce. Reserve the remaining ½ cup barbecue sauce for finishing the ribs.
Arrange the rib sections inside a six-quart slow cooker, placing them around the sides with the meaty portions facing outward when possible.
Pour the barbecue sauce mixture over the ribs, making sure each section gets coated.
Cover and cook on LOW for six to seven hours, or until the ribs are very tender and the meat has started pulling away from the bones. You can also cook the ribs on HIGH for three to four hours.
Line a rimmed baking sheet with aluminum foil. Carefully transfer the cooked ribs from the slow cooker to the baking sheet with the meaty side facing up.
Brush the ribs generously with the reserved ½ cup barbecue sauce.
Broil for three to five minutes, watching closely, until the sauce bubbles and begins to caramelize around the edges.
Remove the ribs from the oven and allow them to rest for five minutes before cutting between the bones and serving.