I just had this exact problem last week. It's very scary, I thought a wheel was about to fall off. I had just come over a rise at about 70 mph, and was on my way down when the vibration started. It had happened once before under similar circumstances. I replaced both tie rods and the idler arm/bushings, and the problem went away.

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Tjohn

82 300 SD
77 450 SL (gone)