Rather than trying to find the mythical singular cause of your woes, I would start with taking apart everything that moves, from the frame to the hub caps. You can thank me later. My favorite brand of beer is cold.
Leaf spring pins and bearings.
Castor shims. (Are they there?)
King pins.
Wheel bearings.
Tierod ends. Check for egg-shaped ball ends.
Steering rod ends. Same as above.
Steering box. If it has leaks and no fluid, rebuild it. (It is not as hard to do as most believe.)
Adjust the steering box according to the Shop Manual.
Adjust toe in.
Carl
