I get the same thing whenever I go to AutoZone or Kragen. Go to AutoZone.com and punch in all of your stuff. Sometimes they give measurements for specific parts. When I went to AutoZone for wheel cylinders I had 3 size choices depending on truck model. When I went to Kragen they looked it up, walked in the back and threw the part on the counter. When I asked if it was the correct out of 3 they said there is only one size and that was it. Don't get irritated, it is more irritating for me to take the truck apart to take measurements, put it back together to drive to the store and come home take it apart again and realize I have the wrong part. It sounds like we have similar trucks and the correct parts for me always seem to be the HD parts. When in doubt take the rotor in and match it up.