There was a thread on caster or the lack of it, it was noticed that the 2wd sierra upright had built in negative trail ( the wheel spindle is forward of the upper and lower swivel positions) . Its this that causes some lack of self centering in the front set up. The off set mushrooms dial out some of it but moving the top mounts back would probably give you better results. The 4x4 sierra uprights have no negative trail.
Bob
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