could just be the type of tires you have on it, but check your tire pressure too, if it has even halfway offroad tires it's probably nothing.. my 4WD has all kinds of shimmies and vibrations that would be alarming if my firebird did it, but is usually nothing, especially if you play in the mud.. get the tires nice and muddy and then get on the freeway... it's quite a shock the first time you do it
