O.K. so it sounds like both of you are on the right track, but...
I have overstock length shackles, BUT! I have had them on for years with absolutely no problems at any speed from 75+ on the Highway, to 35 mph around town to .3 mph on a trail. The problem only started after I replaced my stock drag link and tie rod with an after market beefier set from Big Daddy. They use the same TREs so nothing should have been affected other than my toe-in, which I had to guess and set myself, mainly because the original tie-rod was now shaped like a u-joint and I had to adjust the hell out of it to get me off the trail.
As for the problem, it sounds much like what you described with bump steer. It is only caused at speeds around 35 mph where I hit a rough piece of road at speed. The one side compresses, and the steering wheel jerks, but not once like you mentioned, rather it death wobbles back and forth trying to find center until I slow to under 5 mph or so and give the wheels a chance to calm down and find center again.
I just took a look at my toe-in and found (By measureing from inside rim to inside rim) that I had a good bit more than a 1/4 inch wider in the back. From what I've heard this should be 1/16" to 1/32" so I udjusted it to be closer to 1/16". I'll ride this for a week and see if it helps or hurts.
As for the Caster, where and how do I measure this??? Given that I have longer shackles it certainly could be a problem.
Thanks for the help.
KNIGHT6 - '78 CJ5.
Ford 9" x2, Tru-Hi-9 High pinion, 5.14s, 2 detroits,
SOA, D44 'flat-knuckles' High Steer over springs,
Chevy 5.7L Vortec, Ford T-18 with granny-low 1st
37x14.00 Iroks, Warn 9K lb winch,
Metal Fusion tube fenders,full cage frame tie-in, +++