Check the "preload" adjustment on the steering gear. With the steering centered, there should be NO free play in the steering wheel. Even a slight movement of the wheel should move the Pitman arm. If there's any slop at all, it will shimmy at highway speeds. The service manual outlines an adjustment procedure, or there are several shade tree methods that work pretty well. I use an inch-pound torque wrench on the steering wheel nut, with the Pitman arm disconnected so I'm turning only the gear assembly.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!