Stop and check the lug nuts on you tires. Also, go to the front of your vehicle, about 30-50 feet in front, get down and look at the front tires. Are they perfectly vertical?
You have a lug nut wrench in your vehicle, its with the jack. Use it to see if you can tighten any of the lug nuts. Because that wrench is so small, it is unlikely that you can overtorque the lug nuts, so if they move at all, then get to a shop and have them torqued down properly.
Check the rear wheels too as Costco will not mount two new tires on the rear unless you just bought new tires for the rear. The will move the rear tires to the front and put the new tires in back.