Perhaps this document I found on-line would explain what CapriRacer was talking about, below is an exerpt:
Document ID# 2034415
Subject: Revised General Motors New Vehicle Tire Warranty Program #00-03-10-003M - (10/29/2007)
Models: 1996-2008 GM Passenger Cars and Light Duty Trucks:
The GM New Vehicle Tire Warranty
GM provides it's customers with comprehensive coverage on OEM new vehicle tires. In many cases this warranty exceeds the coverage provided by the tire manufacturers themselves. The GM new vehicle tire warranty program is designed to offer dealers the required amount of latitude to handle any type of tire concern and repair the customers vehicle with a minimum of approvals. The web based feedback structure for reporting and shipping tire returns remains unchanged.
Program Rules and Information
The sections below detail what conditions are warrantable under the terms of the New Vehicle Warranty. All tire warranty replacements made by a GM dealer are without a mileage pro-rate charge.
What Is Covered
Only Original Equipment (OE) tires are covered against defects in material and workmanship. OE tires will have a Tire Performance Criteria (TPC) number molded on the sidewall near the tire size. (Exceptions: Vibe, and Aveo, do not use TPC tires, but are still covered.) Any replaced tire will continue to be covered for the remaining portion of the New Vehicle Warranty. Tires will be warranted without prorated charges for tread mileage.
Important: Tires purchased by consumers beyond the New Vehicle Bumper-to-Bumper coverage or even replaced during the Bumper-to-Bumper coverage for wear out are warranted by the tire manufacturer (including tires purchased through the Goodwrench Tire Program). Contact the tire manufacturer for warranty processing instructions.
Following expiration of GM's coverage, tires may continue to be warranted, on a pro-rated basis, by the tire manufacturer. Review the tire manufacturer's booklet (included as part of the glove box material).
What Is Not Covered
• Non-original equipment tires (those without a TPC number molded on the sidewall (except Aveo and Vibe) are not covered.
• Normal tire wear and wear out is not covered.
Important: Tire wear greatly varies by both the tire type installed and the driving habits of the owner. OEM tires installed on GM vehicles are evaluated for many different criteria and may have been selected to optimize handling, ride quality, load carrying or traction demands. Different types of tires will experience different rates of wear, resulting from all of the above factors. No warranty is offered on the usable tread life of the tires.
• Uneven wear from damaged, worn or mis-aligned suspension components is not covered unless the wear has resulted from a warrantable suspension condition. Tire replacements resulting from these allowable suspension conditions should be included on the same RO#. (If this is not possible the RO# for the tire replacement should reference the RO# the warrantable suspension component/condition is listed on, or be attached).
• Road hazard is not covered. This includes punctures, cuts, impact breaks, etc.
I assume that the fact that GM installed the shim in the rear axle and performed a 4 wheel alignment NO CHARGE would amount to a warrantable suspension condition.
