Place the sliced onion and garlic at the bottom of your crock pot.
Rub the pork with the spices, sugar, salt, and pepper. Set it on top of the onion and garlic.
Pour barbecue sauce and Dr Pepper (if using) over the pork.
Cover and cook on low for 8 to 10 hours, or until the pork is fork-tender and shreds easily.
Remove the pork, shred it with two forks, and return it to the slow cooker. Stir it into the juices.
Serve warm on sandwich buns with extra barbecue sauce if desired.