There may be more to the "a similar truck as mine with the 3.42 rear end has a tow capacity of about 9500#." specs than just different gearing. The axles may be stronger, bigger brakes, higher capacity springs, different shocks plus auxillary cooling and more.
When I was shopping for 3/4 ton trucks the gearing, brakes and springs were different from the factory for the higher tow capacity truck vs. the lower capacity version of the same truck.
That may be the reason the dealer would warn about voiding your warranty. You may need to add more to match the 9500# truck.
