1. Combine salt, chili powder, cumin and coriander; rub all over roast. Warm oil in a large heavy pot over medium-high heat. Sear meat on both sides, about 4 minutes total.
2. Place meat in slow cooker. Mix tomato sauce, chipotle and garlic; pour over meat. Cover and cook on low until fork-tender, about 6 hours. Shred meat, combine with enough sauce to moisten, and serve with tortillas.