Place 3 pounds boneless, skinless chicken breasts in a 4-quart or larger slow cooker.
Top evenly with 1 (1-ounce) packet ranch salad dressing and seasoning mix, 1 (1-ounce) packet au jus gravy mix, 1 stick cubed unsalted butter, 6 whole pepperoncini peppers, and 2 tablespoons pepperoncini brine.
Cover and cook until the chicken is tender and registers an internal temperature of at least 165°F, 4 hours on the LOW setting, or 3 hours on the HIGH setting.
Transfer the chicken to a clean cutting board or large plate.
While the chicken is still warm, use 2 forks to shred the meat into large pieces.
Return the chicken to the slow cooker and toss in the cooking liquid.
Serve with more pepperoncini peppers if desired.
