Wouldn't be surprised if your tires DID NOT have anything to do with it. I have a bone stock Titan (was about to make it un-bone stock), My diff blew on me on 11/30/04.
Is Nissan working with you? They've been screwing with me a bit so I got the Better Business Bureau Autoline involved. I also started the arbitration process for Massachusetts State Lemon Law. State Law says after 15 days I can do this.
According to my dealer and Nissan Corporate I should be receiving the whole read end (brake, axles, diff. everything, an 05, the week of Dec. 27th.
It's ashame, I only had 6,000 miles on it. Who knows ,aybe I'll get an 05 Titan out of it though.