Yes. take a plane, (hand, block, or power) and take your two ends and plane the opposite corners down till both squares sit in the same plane. Try to do this in such a way that it squares you off to another side of the timber if possible. Then take a chalkline and snap a line from one reference square to the other on the vertical side of the timber. Do this on both sides of the timber. If you plane to that line you should get a perfectly flat surface.