Combine paprika, sugar, salt, pepper, onion powder, and granulated garlic. Reserve 1 tbsp of the spice rub.
Pat the ribs dry and coat with the remaining spice rub.
Arrange ribs vertically in the slow cooker, thick ends down, meaty side against the wall. Cook on high for 5-6 hours or low for 6-7 hours.
Whisk together ketchup, apple juice, molasses, vinegar, Worcestershire, mustard, pepper, liquid smoke, and reserved spice rub in a saucepan. Bring to a boil, reduce to medium-low, and simmer until thickened, about 10 mins.
Line a baking sheet with foil, set a wire rack on top. Transfer cooked ribs to the rack, meaty side up, let sit for 10 mins.
Preheat broiler with oven rack 3 inches from broiler. Brush ribs with 1/2 cup barbecue sauce and broil until bubbling and beginning to char, about 4 mins.
Remove ribs, brush with remaining sauce, tent with foil, and let rest for 20 mins.
Slice between bones and serve.