[f5fstop]

You may not like the answer, but I believe the dealer is acting correctly.
From a GM point of view, if the dealer charges GM for warranty work by rotating tires to find the problem, and it is a tire, GM just paid for your aftermarket tires to be diagnosed.
The dealer is not saying they won't cover under warranty, they will not diagnose with non-factory installed tires.
Bebe, evidently goes to a great dealer. But I know of many dealers who will not diagnose a vibration that could be caused by aftermarket tires, and why should they.

I know of people who have gone in with other vehicles with vibration issues, and with aftermarket tires. The dealer diagnosed an aftermarket tire at fault, and when the customer was hit with the bill, they yelled and screamed. They actually thought GM should pay the bill for their defective tire; that they purchased, not GM.