How does it steer around corners? If you need to turn the wheel back manually, then it needs more caster. Mine felt real light at highway speeds, I couldn't hold it straight when it needed more caster. 2 degree is what most start with.
Caster kicks the bottom of the axle forward. Weight over the axle holds it down, the farther out you kick the axle, the harder it is to turn since you have to lift all the weight with the steering box. But, the farther out you kick it, the straighter it will go on its own, think of a front engine dragster, the axle is really leaned back. This also changes the drag link length, and a fixed length drag link will pull the steering box off center. Boxes need to be centered for correct steering. Off center creates odd turning radiuses and no center support steering ( tighter dead center ).
You maybe having bump steer problems. This is where the drag link and axle travel arc's don't line up. If they are off, the axle tends to pull on the drag link on its travels up and down. Since you are holding the steering wheel tight, the only thing the drag link can do is pull on the steering arms and turn the wheels. Thats why its called bump steer, it actually steers going over bumps.
Just having a level drag link is not always the cure. You need to plot out the travel arc of both the axle and steering box. On a large cardboard sheet, measure the axle height, spring hanger height, steering arm height, and drag link height ( mounting point on the pitman arm ). You will also need length from spring mounting bolt to center of axle, and total drag link length, With all these laid out, draw the travel arc of the drag link and springs. You should be able to see the bump steer pretty clear. I forgot you need the distance from the steering arm to the pitman arm. Basically draw out you truck in full size, cut some cardboard patterns, and run it through its travels.
You dropped the axle 4" , Is the steering box still factory mounted? If so the steering box now thinks the truck is fully loaded and squatted down 4". You have used up most of its travel already. GM designed the steering so at normal driving with no load, the truck will drive great, with a heavy load, you are getting at the upper limit of what they think is acceptable. You are way past that just sitting still.
http://www.eatonsprings.com/techquestions/bumpsteer.htmlJoe
