In a small bowl, mix together brown sugar, paprika, salt, black pepper, garlic powder, onion powder, cumin, and chili powder.
Rub the spice mixture all over the pork shoulder, covering it evenly.
Place the sliced onion and minced garlic at the bottom of the slow cooker.
Place the seasoned pork shoulder on top of the onions and garlic.
Pour the apple cider vinegar and chicken broth over the pork.
Cover and cook on low for 8-10 hours, or until the pork is tender and pulls apart easily.
Remove the pork from the slow cooker and shred it using two forks.
Drain excess liquid from the slow cooker, reserving about 1 cup. Return the shredded pork to the slow cooker, add the barbecue sauce and reserved liquid, and stir well.
Serve the pulled pork on buns or as desired.