out of balance

theres a possibilty that your front wheel bearings could be going bad. tha could easily cause your truck to have that "shimmy" your talking about.

I noticed this on my 2007 Silverado at 40MPH and is now doing it 60-65 MPH too. the steering also developed sloppy control and have the intermadiate shaft,idler & pittman and gear box replaced and still no results. I have been told the bearings are fine so what else??? all this at 52K miles.

Just had the same thing with my 1500HD. Changed U-Joint, balanced all the tires, tore apart front transfer case and still vibrated at 40 mph. Had the propeller shaft replaced and now it is all good.