Spray a 6-quart slow cooker with nonstick cooking spray. Cut tenderloin into 3-inch pieces and place in the bottom of slow cooker.
Pour chicken stock over pork. Top with onion, garlic, salt, pepper, cumin and chili powder; mix to combine.
Cover and cook on low for 6-8 hours or high for 3-4 hours.
When pork is done cooking, remove from slow cooker and shred with two forks. Preheat oven to high broil.
Spread shredded pork on two baking sheets that have been lined with foil. Pour 1⁄4 cup of juices from the slow cooker evenly over the pork in each pan and toss to coat evenly.
Broil for 5 minutes, or until the edges of the pork start browning. Serve immediately on tortillas or salad and top with your favorite taco toppings (not included in nutrition information).