The reason for bumpsteer is not because your drag-link and track bar aren't parallel. That just causes binding when they move in different arch's. The reason for the bumpsteer is because the drag-link is not parallel with tie-rod.
Looking at his setup, he'll need to raise the track bar on the axle side to get it parallel with the drag-link. If he lowers the drag-link with a pitman arm, it will be even more out of parallel.
Still, if he's got a locker then it's the locker... You've got to learn to shift when you're going straight and not while in a turn.