Remove the thin membrane from the back of each rack of ribs by loosening a corner with a paper towel and peeling it off completely.
In a small bowl, mix together brown sugar, smoked paprika, garlic powder, onion powder, chili powder, cumin, salt, and black pepper. Rub the spice mixture generously over both sides of each rack.
Cut each rack in half so the pieces fit in the slow cooker. Stand the rib sections upright along the inside wall of the slow cooker, meaty side facing outward, curving them to fit.
Drizzle the apple cider vinegar over the ribs. Cover and cook on low for 7 to 8 hours or on high for 4 to 5 hours, until the meat is very tender and pulling away from the bones.
Carefully transfer the ribs to a foil-lined baking sheet, meaty side up. Brush a generous layer of bbq sauce all over the top and meaty sides of the ribs.
Broil on high for 3 to 5 minutes, watching carefully, until the sauce is caramelized and sticky with slightly charred edges. Let rest for 5 minutes, then slice and serve.