May be tires
Hi
In 30 years of driving CMPs I've had this problem with all three of my trucks. In each case I've done as suggested check setting, looked for play installed heavy-duty steering damper. All good things to do any way, some improvement but no complete fix, but each time changing the tires solved the problem or moved the speed that it starts at well above normal driving speed.
My C60S is the happiest highway truck of the three, even with the radio box the 261 engine and 11:00x20 tires it is smooth at 50-60 MPH ( with 11:00 tires the truck thinks it is doing 50 ) All three of my trucks have recent manufacture new tires. My C60L has a little shimmy as you get to 45-50 MPH which I suspect is from no front shocks which is letting little wheel hop feed in to steering. I suspect that the steering geometry of the pitman arm steering connector link is not perfect that allows the up down motion to be converted to lateral motion in the steering. Will see if this goes away once the shocks are bolted in.
Given my experience would try the following with the wheels jacked up take large block position so it just doesn't touch the tire hold something on the block and slowly turn the wheels to mark a line down the center of the tire. Then look at your line is it staying centered on the tread and did then your marker try to move in and out as you turned the wheel. Assuming the tires are running true an are not egg shaped, then would try moving your tires around front to rear side to side, does the shimmy change?
Cheers Phil
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