Combine kosher salt, chipotle powder, chili powder, garlic powder, onion powder, cayenne, black pepper, smoked paprika, and brown sugar in a bowl. Mix until evenly distributed.
Butterfly each drumstick by making a lengthwise cut along the bone. Open it flat like a book without cutting all the way through. Pat dry with paper towels.
Generously coat each butterflied drumstick with the dry rub on all surfaces. Let sit 30-60 minutes, or refrigerate uncovered up to 4 hours for crispier skin.
Prepare grill for direct heat. Sear butterflied drumsticks 90-120 seconds per side until charred. Then set up grill for indirect cooking at 325°F. Add hickory wood chunks.
Move seared drumsticks to indirect heat. Close lid and smoke until internal temperature reaches 165-180°F, approximately 20-25 minutes.
Combine mayo, sour cream, buttermilk, lemon juice, herbs, seasonings, and diced chipotle pepper. Stir well and refrigerate until serving.
Remove drumsticks from grill. Serve with chipotle ranch dipping sauce on the side or pre-dunk each leg before plating.