Trim fat from meat. If necessary, cut pork to fit a 4- to 5-quart slow cooker. Place pork, onion, and garlic in slow cooker. In a medium bowl, combine chili sauce, brown sugar, cider vinegar, Worcestershire sauce, chili powder, salt, and pepper. Pour over pork in cooker.
Cover and cook on low-heat setting for 10 to 11 hours or on high-heat setting for 5 to 6 hours. Remove pork from cooker, reserving juices. Using two forks, shred pork, discarding any fat. Skim fat from juices. Add enough juices to the pork to moisten. Place 2-cup portions of pork in airtight containers and refrigerate for up to 3 days or freeze for up to 3 months. Makes 6 cups.