Tires for my 06 Explorer?
If the tires are wore down to the marker, they are wore out and are not legal to run on the road now. If you get stopped or get into an accident, you will be ticketed for unsafe tires. Tires that are wore to the legal limit aren't as efficient on wet slippery roads and reduce your ability to control the vehicle safely. Only replacing two will not damage the vehicle in any way. Also as previously noted, rotating your tires regularly will greatly increase your tire life as it helps to keep them wearing even and also wears all four tires at the same rate.
just rotate them(put the fronts on the backs and the backs on the fronts),u can make ur tires last longer by doing so, the front tires will allways be the ones that get worn down first and the most