RE: Front and Rear Differential Fluid Change.
Your maintenance schedule in your glove compartment will tell you to have it changed every 15k miles.
Your differential fluids help protect your gears against wear, and it suspends wear metal particles that aren't big enough to be trapped by the magnet in your differential case. Since most break-in wear will occur during that first 15k, I think it is a good idea to get both the front (if you have a 4x4) and rear changed at that first interval. After that, if you don't haul much of anything, and if you don't race at every stop light, I think it is okay to go every 30k or maybe even longer. That first change is important, though.
There used to be a DIY post by Osteodoc on changing the differential fluids, but I can't find it now. It's actually not that hard, and if you can change your engine oil, then you can do the differential fluid, too.
Warning: This is only my thought on the matter. Not responsible for any damage caused.