A picnic or butt or boston butt which are all from the shoulder of the hog are the best cuts of pork for pulling. When you get pulled pork at your local BBQ joint it came from one or both of those cuts. Most hams are from the hind end of the hog and the aforementioned cuts are from the front shoulders. There are a few Butchers that frequent this site and one will be along shortly I am sure to more specifically tell you why.
I think of a picnic roast as more of a ham. Is it typical to pull this cut?