You could have a defective dash kit. If it doesn't snap in completely, it can do this. Also, if the cage for the deck isn't tightly fitted and secured to the kit, the deck will be able to shift. If everything is absolutely the way it should be, you may want to try using back-strap. It is a flat metal strip, about an inch or less wide, and a sixteenth to an eighth of an inch thick. It should also have a series of circular or ovel holes, uniformly spaced the whole length. You take this material, and attach it to the bolt on the back of the radio. Once attached, bolt or screw it down to a secure brace inside the dashboard. This doubles as an anti-theft device, because the radio is mounted to the body of the truck.