1. In a blender or food processor, puree chipotle chiles with adobo sauce until smooth. Spoon 2 1/2 Tbsp. into slow cooker; reserve remainder for other uses. Add tomatoes, lime juice, onion and garlic; stir to combine. Season beef with salt and pepper and add to slow cooker.
2. Cover and cook on high until beef is tender enough to shred easily with a fork, about 6 hours. Using two forks, shred meat; spoon into a serving bowl with sauce. Serve beef with warmed corn tortillas, shredded lettuce and cheese, sliced avocado and sour cream, if desired.