I was reading the limited warranty linked to by powerboatr and found the section disclaiming responsibility to damage related to after market parts. This should answer a lot of the "will a custom exhaust void my warranty" questions. My reading of this document leads me to the answer that no, it will not void your warranty. Instead, if Ford determines the after market product caused the damage, they do not have to fix it under warranty. Refusing a particular repair and outright voiding are two very different things. They will have to prove it caused it but then you will have to force the issue when they initially say no. The onus will be on the truck owner at first to force the issue.
from the warranty document:
Damage Caused by Alteration or Modification
The New Vehicle Limited Warranty does not cover any damage caused by:
• alterations or modifications of the vehicle, including the body, chassis, or
components, after the vehicle leaves the control of Ford Motor Company
• tampering with the vehicle, tampering with the emissions systems or with
the other parts that affect these systems (for example, but not limited to
exhaust and intake systems)
• the installation or use of a non-Ford Motor Company part (other than a
certified emissions part) or any part (Ford or non-Ford) designed for
off-road use only installed after the vehicle leaves the control of Ford
Motor Company, if the installed part fails or causes a Ford part to fail. Examples include, but are not limited to lift kits, oversized tires, roll bars,
cellular phones, alarm systems, automatic starting systems and
performance-enhancing powertrain components
from the warranty document:
Damage Caused by Alteration or Modification
The New Vehicle Limited Warranty does not cover any damage caused by:
• alterations or modifications of the vehicle, including the body, chassis, or
components, after the vehicle leaves the control of Ford Motor Company
• tampering with the vehicle, tampering with the emissions systems or with
the other parts that affect these systems (for example, but not limited to
exhaust and intake systems)
• the installation or use of a non-Ford Motor Company part (other than a
certified emissions part) or any part (Ford or non-Ford) designed for
off-road use only installed after the vehicle leaves the control of Ford
Motor Company, if the installed part fails or causes a Ford part to fail. Examples include, but are not limited to lift kits, oversized tires, roll bars,
cellular phones, alarm systems, automatic starting systems and
performance-enhancing powertrain components