Try raising your front wheels off the ground by jacking up your front axle. Should be able to do one side at a time.
With the front tire off the ground, grab the top and bottom of the tire and rock it to see if you have any play. If you do, there is a good chance that the front bearings are loose and need to be tightened. And if they are loose, then they probably need to be re-packed with grease and checked for wear.
Also with it raised, you can check for a bent wheel.
That little damper will not cause the shakes, but a non-functioning one will not prevent the shakes which is one of its main purposes. Drop one end and see if you have an equally stiff resistance in both directions. Also make sure that you have the tires inflated to about 25psi. Might also try rotating your tires to put the front ones on the back to see if that helps. Sometimes a broken belt in a tire will cause a bad vibration or bounce.
Here is a link the FSM Suzuki Sierra SJ413 Service Manual
. Sections 17 & 18 cover the Suspension and Steering.
The spindle nut requires a 54mm or 2 1/8" thin-walled socket, which is not any easy thing to find.