On mine, it was related to week springs and weight transfer. Just replaced both front coils last week, truck is now within 1/2" side to side and no longer pulls to the left under hard braking, may even want to pull slightly right now.
Going to use Alcan or other recomendation for rear, remove lift blocks and use 3" lift springs to bring my back down a bit (tow 5th wheel and just got bigger then stock wheels / tires. )