1. Warm oil in a large skillet over medium-high heat. Season chicken with salt and pepper and add to skillet in a single layer (work in batches if necessary). Cook, turning frequently, until golden brown, about 5 minutes. Remove to a slow cooker. Repeat with remaining chicken.
2. Pour off all but 1 Tbsp. fat from skillet. Add onion and cook, stirring frequently, until softened, about 3 minutes. Add garlic; sauté 1 minute longer. Add barbecue sauce and peanut butter. Stir, scraping up browned bits on the bottom of the pan, until slightly thickened, about 2 minutes. Pour sauce over chicken and stir to coat.
3. Cover and cook until chicken is cooked through, 2 1/2 to 3 1/2 hours on low. Uncover, set temperature to high and cook, turning chicken frequently, until sauce thickens, about 20 minutes. Serve.