How much pork are you keeping warm and how many people are you feeding? The reason I ask is if I have cooked multiple butts, I only pull them one at a time as they are needed. The others remain wrapped in foil in a cooler.
How long are you keeping the food out? A few hours can get dangerous if not kept at proper temp.
To keep pulled pork warm, I have used all of the above methods. Also, an electric griddle set on warm with an pan on top works.