Caster is the tilting of the steering axis either forward or backward from the vertical, when viewed from the side of the vehicle. A backward tilt is said to be positive and a forward tilt is said to be negative. Changes in caster affect the straight line tendency of the vehicle and the "return to center'' of the steering after a turn. If the caster is radically different between the left and right wheels (such as after hitting a sizeable pothole), the vehicle will exhibit a nasty pull to one side.
On the Pick-ups and Monteros, caster is fully adjustable, either by adjusting the upper arm cross-shaft (Montero) or by changing shim thickness (Pick-up).