Even when there's some relationship between the mod and the damage, say new, larger control arms (what I'm running) and a blown CV, the dealer should still consider seriously the possibility that it's just actuially a bad CV. A diff that blows "because" someone's running 33s? We at thenewx now know that that same diff could well go for someone running stock 265 75s or 265 70s. And it's Nissan's fault, even if the rig is driven pretty hard. Some of the most experienced drivers on this site have blown diffs.
We shoud take the friendly but firm line at the dealer that when we're modded, we're a driving advertisement for the X, we advertise the vehicle everyday on our forums, and many of us treat it with more loving care than a Nissan tech. does. And stronger control arms or bigger tires, within reason, make the X a better truck, plain and simple. Diffs ,and clutches, and driveshaft yokes should be up to those sorts of quality mods, and are in many vehicles around the world.
If I have a stock front end and I blow an original shock after hitting a big pothole with my load of a Shrock bumper and winch weighing down the front, well...that's my bad. I'm pushing the shock beyond tolerance. But lots of this crap the dealers try to pull makes way less sense than that and can be successfully challenged by a confident, knowledgable customer.