Instead of a 2x4 which can be a bit warped, use mason line. Drive the truck so it's driving straight and leave steering wheel in place.
I use a jack stand to tie the mason line to. Put it behind the rear wheel, Raise it to the center of the rear tire and pull across the face of both rear and front tires. With the line touching the front tire, back edge, the front edge should have a gap of approx 1/8" on the front edge. Check the other side as well. An 1/8" gap per side, or toe in, is just about alignment spec.
Easy way to immediately see how far off you are.