I have a 98 Jeep Wrangler 4 cyl. When I drive at higher speeds (40 mph or greater) the front of the jeep shimmy's when I hit a bump. It feels as though the tires are not keeping in contact with the pavement. I know this will not be something I can fix myself but would like to have some idea of what could be wrong as to not get rip off getting it repaired. Any advice would be great.