Is your drag link original? I had a similar issue with a 3100 that installed CPP power steering kit in. The only way to fix it was to adjust the length of the drag link to equalize the left and right turning radius.
I know that my example is comparing apples to oranges, but the length of the drag link, or an out of position pitman arm are the only two items i can think of that can cause your issue. Unless the truck has been in a collision at some time in it's past and the frame or something else is bent.
