From what I have been able to learn, you probably shouldn't waste any time or lose any sleep over how long your differential is going to last. It seems, that for the most part, if they are properly maintained they will likely last as long as the rest of the car.
My original was replaced at about 50,000 miles when the seal began to allow a very slow drip. Because I used the car on the highway a great deal, I feared that the seal might give way completely, dump all of the grease on the highway and leave me stranded. Hindsight tells me I should have run the risk. I wouldn't have been any worse off than what actually happened.
Prior to the failure, there was absolutely no noise in the differential. I only wish that there had been a Steve around to take care of it for me. The nut was torqued so tight that it took a long breaker arm to loosen it. The man who did the job obviously didn't know what he was doing.
Good luck with yours!