Well, you do have the rears on a different path/track than the fronts. Sometimes big tires do like to follow ruts/depressions/crack/etc in the road. I wonder if flipping the hubs and reversing the rims to make the rears track the front would help.
Overall, a good time to go through everything to check if there's slop. Make sure when you are checking your steering components, you have the axle housing on stands to take pressure off. Loose wheel bearings, slop in tie rods, slop in the steering box, improper caster, improper toe-in, runout in the tires, etc, all can be causes. Worn suspension bushings as well.
Did you check the run-out of the tires? Thats easy to do without tools:
jack up the axle and put it on a stand just so its off the ground and can spin freely. Get a cinder block or similar, put it on the ground by the sidewall. Put a dowel, rod, screwdriver, whatever, on the block, so it just barely touches the sidewall, and put another heavy weight to clamp the rod in place. Now spin the tire, and see how much its shimmy-ing side to side. Find your two best tires and put them in the front. (note: I believe you just have a certain amount of sidewall ripple with these tires as well....)