Yep, I just used one pneumatic cylinder. It's pretty much centered, but turned out to be a happy coincidence that it must be just slightly to the rear of center and ended up rocking when triggered.
I used a 5 port, 4 way valve, and a "Nerve Center" controller from MonsterGuts.com
When I was creating the routine via the controller, I didn't always just poke the buttons, I would hold the button in for a second and then release, this gave me the rocking motion when needed, and when I wanted more jumpy actions, I triggered it more quickly.
As opposed to what I figured most people do, I actually created the routine while everything was already mounted in the box (with the lid open). That way I could really get a "feel" for how the routine was going as I was programming it.
Hope that helps. If you need more info, just let me know.
Roo