I have a 1984 Ford F-150 4x4 that has a steering problem. While iI am driving it will star to pull to the right, when iturn steering wheel to correct it, it catches up and jerks the truck to the left. I have replaced the axle pivot bushings, radius arm bushings, all worn tie rods, and wheel bearings. The gear box is fine as are the brakes.All ball joints are fine too. Any ideas on what could be wrong?

Optional Information:
1984 Ford f 150 300 6 cyl