Ok, now I see that you already knew it was an aftermarket spring (Deaver, from a previous post) but that you were wondering if the number of leaves is making it ride stiffer, or having any other negative effect on the handling?
The answer is that, in most cases, the newer 11-leaf packs ride better than the old 5 and 6-leaf packs did.
They are not "soft riding" by any means. But they are less "harsh riding" than the old springs. However even the good springs can settle down and ride worse than when they were knew. My 2.5's are not exactly Cadillac smooth by any stretch!
But they're not likely to be causing you to experience the wandering, unless their bushings or mounts are wearing out.
Even though not all, most wandering comes from the front.
I don't see where you did any other testing. Frankly, even your relatively low caster readings from the first post are not low enough to cause your problem. They're better than mine and I can run 80mph in a very steady fashion. I may not do it very often, but I can when I need to. And both of my caster readings are less than yours.
Tires, loose components, and incorrect comparisons/assumptions are perhaps the most common culprits.
So first of all, you need to do "the test" we used to call it, where with the tires fully on the ground, a helper racks the steering wheel back and forth about a half a turn in each direction while you crawl all around and under the rig to see if anything is moving that should not be.
If you're not sure what should, or should not be moving when doing this test, let us know and we can list down all the usual suspects.
And just as important as the more detailed testing, are you familiar with old trucks and utility vehicles? Or is all of your previous driving experience with newer vehicles? If newer only up until you got the Bronco, then perceptions are worth discussing.
If you've been around older vehicles all your life on the other hand, then you're familiar with how they should feel.
Either way still leaves us with the same things mentioned before. Tires type, tire quality, tire wear, tire damage, tire age, tire wear, bearings, ball-joints, u-joints, trackbar mounts, trackbar bushings, the other steering components, lockers and limited-slip differentials, rear wheel bearings, and tires.
Did I mention tires?;D%) Yeah, I know... But they are pretty important in this discussion.
Good luck.
Paul