Caster - the rearward leaning of the steering angle of each wheel is what helps pull the wheels back to center. When the wheel is turned it actually rolls up on one side of the tire. So when the steering wheel is released, the tire wants to roll back to flat, and this pulls the steering wheel center and straightens the car.
All that being said, the caster is probably NOT causing your issue - both measurements would have to be practically 0, which would only be possible from some kind of collision or pieces flat-out breaking, at which point the front frame would probably be dragging on the ground anyways.
I'd suspect the steering gear to see if it's seizing at a certain point, or if any of the arms (pitman/idler) or balljoints/tie-rod ends are sticking. Your best bet is going to be to get the vehicle on a lift with someone at the wheel and have them steer back and forth to check for worn or sticking parts.
Jeff
