Persistent bump steer

I've done so much to this thing to try and get the ride right as well as keep the lifted look. I understand a lifted truck is going to wander a bit, but this thing will almost dive off the road if I take my hand off the wheel going down the highway. It's a chore to drive for more than a short distance because I have to basically fight the steering wheel to keep it straight. It isn't pulling as far as I can tell, it just follows the slope of the road very sensitively. There doesn't seem to be any play in the steering and it responds well... just sensitive. The suspension has had time to settle since I've been running it for close to a year.

What I've done:

1. 2" Ironman lift (leafs/coilovers) with 3/4" aluminum spacer up front to level.
2. LR UCA set to neutral postition (+2 caster).
3. New wheels and tires (255/75/17 @33psi), road force balanced, 17X8.5 6-139.70 -6
4. Alignment done:
-- Caster = 2.9 and 2.8
-- Camber = .1 and -.1
-- SAI = 11.9 and 12.2
5. No extra weight in the front or rear.
6. "Zip tie mod" which shouldn't make a difference on the road.

I honestly don't know what else to do. I've just taken the tire pressure down to 33psi to see if that helps at all and I've been looking at replacing the steering rack bushings as a last resort. I literally don't know of anything else it could be.

 

17X8.5 6-139.70 -6

What's a "negative scrub radius"?

 

So if I'm calculating this correctly(8.5" width and -6" offset), I have a 4.5" backspace right? Isn't that where it's supposed to be?

If caster is the issue I'll max it out, but I'd like to be sure before I take it in for another alignment... again. They aren't cheap and I don't think "I didn't have my caster, toe, or camber numbers correct. Can you do it again for free?" will fly.

 

Wait, does the scrub radius have anything directly to do with backspacing or are they just sort of related? Adding a 1" spacer would increase my BS to 6.5, correct? Which would solve the scrub radius problem (if I have one), but throw off my backspacing.