Place onions in crock pot. Blend brown sugar, paprika, salt and pepper; rub over roast. Place roast in crock pot.
Combine vinegar, Worcestershire sauce, red pepper flakes, sugar, mustard, garlic salt and cayenne pepper and stir to mix well. Drizzle about 1/3 of mixture over roast. Cover and refrigerate remaining mixture. Cook roast on low for 8-10 hours. Remove meat and onions, discard onions and shred meat. Add juices from crock pot and remaining vinegar mixture (a portion may be reserved to pass). Serve plain or as sandwiches.