In a small bowl, combine brown sugar, salt, onion powder, garlic powder, pepper and paprika. Set aside.
Remove ribs from their packaging and pat dry. Look at the back side of each rack of ribs. If the membrane is still in tact, remove it by sliding a dull knife or your fingers underneath to loosen and peel it off. Rub the spice blend all over the meat on both sides. Cut racks into 3 even sections each.
Crumple foil loosely into about 5 balls the size of your fist. Place these on the bottom of a 6 quart slow cooker. Stack ribs on top of the foil. Make sure you able able to secure the lid all the way and they are not touching the lid. (In my 6.5-quart slow cooker, the sections of ribs fit tightly).
Cook ribs on low for 8-9 hours on low or 5-6 hours on high. (TIP: low and slow is recommended).
When ribs are fork tender, place them on a foil or parchment paper lined sheet pan and brush with barbecue sauce. Serve immediately, or proceed to the next step if desired.
Turn your oven's broiler to high and place a rack near the top. Place ribs under the broiler for 3-4 minutes or until sauce is caramelized. Watch closely so they don't burn. You could also opt to grill them on an outdoor grill over high heat for several minutes.
Cut into individual pieces and serve immediately.
Serving size will vary depending on how many ribs are in each rack that you purchase.