Here is picture from GM showing the difference in '36 and '37 truck steering. Pay attention to the compression and rebound walk. If you end up with the drag link not on center of its theoretical path, you can end up with bump steer. This why everyone says the drag link must be level.
To explain caster angle, picture a shopping cart, it will easily go any direction you point it because of negative caster. Now picture one of the old time dragsters with the front axle laid way back, they would go dead straight almost by them selves because of all the positive caster. You enough want positive caster to go straight with little input, yet still be able to steer.
Toe-in adjustment causes similar driving problems. The tires just don't know which way to go so they hunt around for a straight path to follow. In actual driving, the tires should be dead ahead straight. The toe-in as set for the front of the tires ( at spindle hight ) to be 1/8" closer together and rear of tires 1/8" farther apart. When you are driving, all the slop in the steering joints is compressed and the tires end up dead ahead.
