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Point the wheels straight ahead. Completely remove the drag link. Find the center of the gear box, count turns left to right and put it dead on center. If your wheel is off then someone installed the wheel incorrectly and you will need to fix it. Index straight ahead with a piece of tape if it's off so you can use that as your center point and you can fix that after. With the wheel centered from left to right and the tires pointed straight ahead adjust the drag link to the correct length by turning the tie rod ends equally in the sleeve until the drag link is the proper length to install. If you need to re-clock the wheel, do so and then adjust the drag link to get a perfectly centered wheel.
And is it safe to assume that you made sure the frame isn't cracked behind the box?
That's called Death Wobble. It's more than a little scary. Steering box play, like your "dead spot", will do this. Check that the following are not loose;Steering box play (you're already handling this).Top and Bottom Swivel Ball Joints between the knuckles and the axle ears.Steering rod joints on the Draglink from the pitman arm to the RH knuckle Steering rod joints at the tie rod ends on the RH and LH knuckle.Wheel bearings. Dana 760X axle shaft universals.Spring to axle U-bolts.Toe, camber, and caster being out of whack will give you death wobble but if anything is loose you will never be able to properly adjust the toe. Camber and Caster are not adjustable they should only be out of whack if the axle is bent.Lift kits without all the neccesary steering mods will cause Death Wobble as well.Lift kits without a properly lowered pitman arm causing extreme angles on the draglink ends.Lift kits with out modified draglink causing extreme angles on the draglink ends.If you do wheel bearings or axle shaft universals don't forget the axle shaft to spindle bearings and seals... Oil Slinger, Outer Axle Seal, Spacer, Axle Bearing Seal, and Axle Needle Bearing. Usually one or more of these pieces are missing or extremely worn out.The above five pieces are #48-52 on the Dana 44 parts blowup here--->1973-80 Dana 44 & 1977-83 Corporate "10 bolt" front axlesDon't beat on the spindle directly to remove it. You'll end up with an oval spindle.DO NOT drive a lifted truck above 65-70 MPH.