The owner's manual for my '06 b2300 states that the rear diff lube doesn't need to be changed unless the rear diff is submerged in water. I'm going to be doing it anyways, though. I don't believe in "lifetime" lubricants. Back in the late 80's, Volkswagen claimed that the fluid in their manual trannies never needed changed, and even went so far as to put tamper-resistant drain holes on them. However, the fluid was hardly "lifetime", and there were numerous reports of manual trannies failing as soon as 50k. Used Oil Analyses done on "lifetime" fluid changed out after less than 50k were absolutely horrible, as expected.
Needless to say, I don't buy into the whole "lifetime" lubricant craze. If it's used to lubricate moving parts that generate heat and friction, it will need to be changed, eventually.