Rub pork with salt, pepper, and garlic. Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add pork; cook 3 minutes on each side or until browned. Transfer pork to a 5-quart round electric slow cooker coated with cooking spray. Add stock to pan, scraping to loosen browned bits; cook 2 minutes. Pour stock mixture over pork; add water. Cover and cook on LOW 8 hours or until tender.
Remove pork from slow cooker, and place in a medium bowl. Pour broth into a medium skillet. Bring to a boil over high heat; boil 5 minutes or until reduced to 2/3 cup.
While broth reduces, shred pork using 2 forks. Stir in broth reduction.
Warm tortillas according to package directions. Serve pork with tortillas, sour cream, if desired, and Chimichurri Sauce.