Arizona has a higher tax on heavy duty diesel than on the light truck diesel. You can still use the large commercial truck pumps then you have a choice to make. Go inside and wait in line and explain you used the large truck pump and the station by law must refund the tax difference (right now 8 cents/gal). Or you can just pay the extra tax and be on your way. As for it to be illegal the only thing illegal is for the large commercial trucks to use the light truck pumps.
In Arizona you will see prices posted as heavy diesel and light diesel. I had a friend who claimed his dodge got better MPG on the higher priced heavy diesel. The only difference in the two is the tax. At most stations the fuel for both comes from the same storage tank.