Hang on a minute... I hate to contradict you but that sounds a bit misleading. The Magnuson–Moss Warranty Act makes it illegal in the US for a company to void their warranty simply because you've installed any aftermarket parts. All they are allowed to do under the act is deny a claim against the warranty if any of your aftermarket parts are the cause of damage to factory parts. It may sound like semantics, but it's an important distinction.
A good example that was mentioned on here a few times was someone that had a poorly installed aftermarket CAI from Mopar that allowed dirt to enter the engine and ruin it. The repairs to the engine were denied under warranty because if it weren't for the CAI, the damage wouldn't have happened.
I haven't heard of a similar situation with an axle, but I'm sure it is possible. Anyone have examples or experience for this kind of thing? I think that's an important discussion in and of itself, because it applies to these axles the same as Dynatrac's or anyone else's. The only situation I can think of is they might try to deny repairs to the transmission or transfer case due to extra wear and tear caused by other drivetrain components being beefed up, but that would be a really flimsy argument. Even if they tried something like this, they would have to prove that your axles are the cause if you decided to fight them in court, and unless the parts failed way before their engineered life expectancy I don't think they would have much of a case. At worst, they would flag the vehicle in their system which would cause them to highly scrutinize any warranty claim you make (effectively letting them give you a run around), but unless they can decisively point to your axle as the cause they'd still have to cover the cost.
Honestly, you guys in the US are pretty lucky to have a Federal law like that. Up here in Canada our consumer protection laws for warranties vary by province / territory and don't tend to be as strong.