Out of round tires, it happens with my dynamics thatt he PO had on there (and there like brand new so I wont bother getting rid of them) or its out of round wheels.

Have them balanced and rotated and see if at low speeds they are fine before warm up and at high speeds before they warm up. Then warm'em up and see how they do.

1996 T100, Mickey Thomson Classics, Bridgestone Dueler AT 265/75/16, custom offroad KC's.