I'm not sure how many times I will have to say this. TUNERS do not VOID warranties.
And, it doesn't mean they will fix it without forcing you go to court either. Manufacturers have a battery of lawyers and freely use them. First rule of lawyers: deny, deny and deny. Second rule: He who has deeper pockets usually wins. Third rule: Baffle them with BS from experts.
Most people don't have the resources to fight a major legal battle. Hell, many manufacturers will fight you EVEN when the customer is right, and its not until a group of consumers can get a class action lawsuit together that you finally get any satisfaction.
Ask a lot of owner of Ford trucks in the late 1980s and early 1990s who's paint flaked off and Ford originally refused to fix the problem. Only when a group got together all claiming the same problem did Ford finally step up and fix what was their problem. And it isn't just this one incident. I just recall it vividly because I had a number of buddies with new late 1980 and 1990 Ford trucks; and they went through hell to get truck repainted by Ford ... and only after a major class action lawsuit was filed.
So, even when you are 100% in the right and the manufacturer SHOULD fix a problem ... often they don't and deem it easier to fight customers legally and avoid a recall or warranty claims.
Manufacturers don't have to do anything.
And, sometimes the manufacturers have "unofficial recalls", where they are aware of a particular issue and will fix a customer's vehicle but ONLY after the customer has bitched enough.
Maybe your dealership and/or Nissan plays nice. BUT not all of the manufacturers do, thanks to the lawyers and the bean counters they have on staff who calculate the cost to fix ... not just one customers vehicle but a lot of vehicles, and wish to avoid a recall at all costs.
PS: I stand by my earlier recommendation to the OP "Put it back to stock, take it to the dealer and play dumb". I think his racing in 4WD with the E-Locker on dry pavement is the bigger issue than using a Tuner. Especially when the Owner Manual specifically says you are not to do that.
I just hope it's a cheap and easy fix for the OP if he cannot get Ford to cover it.