1. Remove skin and excess fat from pork. Make spice rub: Combine all ingredients and rub all over meat. Place pork on a large plate, cover with plastic wrap and refrigerate for 8 hours or overnight.
2. Make sauce: Combine all ingredients in slow cooker, place pork on top and cook on high for 5 1/2 hours, until meat is tender and can be shredded easily with a fork.
3. Make pickled onion: Stir sugar and salt into vinegar until dissolved; stir in onion. Cover and refrigerate for 2 to 3 hours, stirring occasionally.
4. Remove pork from slow cooker. Remove 2 cups of sauce. Shred meat. Stir pork back into slow cooker, cover and cook for 15 minutes.
5. To serve, split rolls, top with meat and pickled onion. Serve, passing reserved sauce separately.