Rearend replacement

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Common practice with GM when the launch a new rear end or any large component such as trans, engine, transfer case. Replacement for up to the first six months, then rebuild from then on. That way they can analyze the problem prior to a technician taking it apart.
This includes things such as a minor case leak, or a solenoid in many instances. GM engineering along with the supplier will study the component prior to disassembling, and that way they can learn, and have the components modified to prevent the failure.
We have done it on all new engines and transmissions including the Ecco tech 4-cylinders, the 5-cylinder in this vehicle when first launced on other vehicles, etc.
If the ID numbers are close, this could be an isolate instance for a particular run of rear ends, and these type of problems are almost always due to a supplier or a third tier supplier (supplier to a supplier) having a quality spill.