Combine tomatoes, jalapeño pepper, garlic, chili powder, and salt in a blender. Pour the tomato mixture into a slow cooker. Add chicken breasts, stir to coat.
Cover and cook on Low for 5 to 6 hours or on High for 2 ½ to 3 hours.
Remove the chicken and place in a bowl. Shred the chicken using 2 forks. Add enough sauce mixture from the slow cooker to the shredded chicken to moisten. Serve the chicken in tortillas, if desired, serve with guacamole, cilantro, and/or lime wedges.