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I've been pretty-much a life-long professional mechanic (incl 20 years w/Ford, prior to retirement), with only a few brief "detours" for other professions, but I've never been an alignment tech. From my trade-school training, however, I recall that (assuming equal tire inflation) a vehicle will "pull" towards the side of most negative caster and/or most positive camber. Soooo......having 'em, relatively even from side-to-side is fairly important. I also recall that the caster specs for most old-school vehicles equipped with manual steering called for it being more negative than their brethren counterparts having P/S.
This difference in caster specs was due to the fact that negative caster reduces steering effort - which helped with those vehicles not having power assist. However, it also reduces the desirable "self-centering" feature.....i.e., a vehicles' tendency to return the steering wheel to a centered position after a turn, AND the stability that reduces "wandering". As pointed out by some, while it has a few undesirable characteristics, negative caster reduces steering effort. Soooo......if
increased effort is desired, adding positive caster (king-pin angle tilted rearward from vertical at the top) should help slightly.
On Ford's twin I-beam suspension, there's only one "factory" method that I'm aware of for adjusting caster and camber on their 2WD trucks.....namely the eccentric on the top ball joint. While that adjustment is primarily intended to adjust
camber .(as pointed out by Walt).......the resulting
caster .can fall into two different settings when camber is correct. This being due to the ball-joint pivot stud being placed toward the front or the rear. When proper camber is achieved with the stud toward the rear......it will be in the position yielding the most-positive caster (what you apparently need). Likewise, when proper camber is achieved with the stud closest to the front......the caster will be more negative.
Other than those two choices, caster cannot be adjusted independently of the camber, AFAIK......without intentionally twisting the I-beam(s) or bending the radius arm(s) ~ requiring a LOT of heat and specialized equipment.
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