Add broth to a 5-qt. slow cooker. Mix seasonings; rub over all sides of pork. Place in slow cooker. Add onion and garlic cloves. Cook, covered, on low 8-10 hours, until meat is tender., Remove pork; cool slightly. Strain cooking juices, reserving garlic cloves; discard onion. Skim fat from cooking juices. Mash garlic with a fork. Shred pork with 2 forks. , Return cooking juices, garlic and pork to slow cooker. Squeeze lime juice over pork; heat through, stirring to combine. Layer tostada shells with pork mixture and toppings as desired. Serve with lime wedges.