brake rotors

[MIREALTOR]

I need to replace the rear rotors on my 2001 2500HD silverado, The local NAPA parts guy says there are two sizes for this truck with no way to tell which one I need before I remove it. The problem is that this is my daily driver and to tear it down and wait for the right parts will not work. It seems there should be some easy way to determine if I need the 4.63in center or the 4.84in any help would be great.

 

On another note is seems they are making the rotors really cheap these days, I have never had to replace them this soon on any other truck I have owned. The pads are not even worn out yet!