I can't say I've noted that winds make my 4th gen feel unstable... I've driven across Saskatchewan and didn't notice it was any more or less squirrelly than any other vehicle would be.
I'd wonder if a bushing might be done somewhere in the suspension and allows lateral movement whenever force is applied to the side of the truck... yet the bushing is sturdy enough to pass the quick inspection a shop would give prior to an alignment.
If the truck feels like a go-kart in a parking lot, chances are it's caster, sure; the steering wheel is getting a tonne of leverage on the wheels so it steers super easy. Consequently, it feels nervous/wobbly at higher speeds. And the truck would feel very unstable in windy conditions because the truck doesn't track to centre on its own, easily; thus inputs can "overcorrect". When i take my hands off the wheel, the truck drives straight as an arrow. With caster out of spec, it probably wouldn't (nor would anyone do that 'cuz the truck is nervous already).
There's a lot of other alignment issues that can cause it, though; I guess what I'm trying to say is look for all other symptoms of caster issues before blowing up money trying to fix that problem... when the problem is really somewhere else:
-feels like a go-kart
-doesn't confidently track straight on a straight/smooth/level road
-steering inputs feel "twitchy", like if you breathe on the wheel it'll turn. obviously the truck shouldn't feel like a battleship to turn, but it's not a smart car, the wheel should give some feedback through it about what the tires are doing.
Get up on the running boards and get the truck bouncing. It should settle quickly when you stop bouncing the suspension, and it should be reasonably quiet while the suspension articulates. If it howls, creaks, squeaks, bangs, something is wrong in there somewhere.
That's all I can think of, hopefully something in there helps. Best luck!
Edit:
Is there any slop in the steering with the car off but key turned enough to move the wheel back and forth? A loose steering shaft u-joint or rack bushings will make it feel bad at higher speeds.
great point, rack bushings being shot gave my truck some pretty weird steering characteristics. All the shops missed it, as well. It occurred to me on day driving it, when the truck veered one way going up a hill, then veered the other going down the hill... seemed obvious it was the rack bushings and I felt real dumb that it took that long to figure it out. As Mike said, have someone turn the wheel with the truck off, you'll see the rack wiggle back and forth on its own. Should be ZERO play in that puppy.
