DGR,
I feel for you....I had the same situation on my K5 after going to a SOFT 4" spring up front.
Over bumps, the truck would pull hard to the right, and then as the suspension came back up, it would pull left. Like you said, almost impossible to keep in your own lane on the highway....and absolutely miserable on long trips. I was bracing myself every time I saw a dip in the road coming up...!
One partial fix for me was to install a front sway bar. 1st Gen Blazers never came with one, but I fabbed up a couple of brackets to hang the later model one, and it helped bumpsteer quite a bit.....not COMPLETELY gone, but good enough that I could take it on long trips and relax a little more when driving.
My eventual solution was going to crossover steering....that was a miracle in terms of fixing bumpsteer for me. There will be people here who argue that crossover steering WON'T fix bumpsteer, or that it's worse than stock steering.....but let's be honest, a lifted truck isn't "Stock" anymore...so it's not a fair comparison.
Anyway, the proof is in the pudding, so to speak and I know that now I can take my hands off the wheel when the truck hits a bump in the road, and it tracks perfectly straight! It's also a lot better for offroad driving too.
Crossover is an expensive way to sure bumpsteer, so if you don't have a swaybar hooked up try that first....a lot cheaper, and it might be "enough" for now. /forums/images/graemlins/cool.gif