Crabbing happens any time your rear axle isn't directly on the center line of the truck. Simple physics dictates that the rear axle must fall into line behind the front, and when it does, the body will appear crooked to the direction of travel. Bad panhard geometry can easily cause crabbing, though you won't know if that's the only cause until you get it corrected and see if the crabbing stops. It won't be caused by the ball joint - that can throw your alignment off, and you're screwed if the ball joint falls apart while you're rolling, but crabbing is a rear axle issue.

Report back when you've got it figured out, we need more aussies on the board. You guys have some nice trucks down there.