hmmm, I guess I'd have to disagree with the gear being so important to get the right PH's. It's really all in the technique. When you know how to do it right, you can get very nice PH's even without any distortion. I'd advise to experiment with different ways to touch the strings (in the right spot!) while picking, f.e. with thumb, index finger, some other right hand finger, finger nail, on down strokes and upstrokes, until you find the way that is most comfortable for you to get the right squeals. In Zakk's case, the vibrato is a major part of his PH sound: make sure it's very wide and has the right speed. It may help to tune down such that the strings are less tense, gives a more flappy vibrato :)