Hey all, this is my first thread, so be kind if I screw something up!! It's great to be on the site! Now, here is my problem. 15K on my 07 4.6 SC with 6.5' Box 4X4, it's been a great truck, but... When I go over small to medium bumps at certain speeds (20-30MPH), the rear box starts this side to side harmonic vibration that is violent enough to make the tailgate start to rattle. Sounds funny but it's almost like some imaginary force is twisting the box on it's center axis. I can watch the movement in the rear view mirror, and would guess it moves at least one inch up and down, alterating side to side at a high rate, almost a "vibration", that takes 2 seconds or so to stop, then start again on the next bump or ripple in the road. I'm afraid my tailgate won't handle it much longer. I don't know if this is just some weird harmonic vibration that the engineers did not work out, or a shock/spring issue. I have read one thread talking about a vibration dampener mounter on the left rear spring shackle, but have yet to see one on a dealer lot. Any thoughts??