Land Rover Repair: Differential Noise / potential failure, warranty company, amunition

Question
Vehicle is a 1999 Range Rover 4.6HSE with 75,000 miles - the vehicle has never been taken off-road.  I have an extended warranty coving the vehicle to 100,000 miles.  I am hearing a rather loud "whirring" / "whinning" noise coming from the front differntial - the noise occurs only when the vehicle is in motion.  The "pitch" of the sound changes with speed (more high pitched the faster I drive).  It is most easily heard when I take my foot off the gas and de-celerate.  The noise is getting worse and most recently, I feel a slight vibration in the steering wheel.  The dealer says that: "it appears that there is no question the diff is failing".  However, the dealer and the warranty company are claiming that they will not repair/replace the front diff until it fails - the noise is not enough.  From what I have read, front diff failures appear common in RR's.  My fear is for my safety and that of my family.  When a front diff fails - does it "lock up"?  can you still control the vehicle? does it just "spin free" allowing the driver to coast to a stop?  Could this noise be caused by anything else?  I need some advice and some amunition to "push" the warranty company & dealer to repair what I percieve as a very serious and potentially deadly situtation.  Waiting for a complete failure seems grossly negligent.  I would like your opinion & advice.  Thank you in advace for your time.

Answer
OK the dealer is stupid very stupid this is very dangerous game they're playing, its called chiken, it could lock up and on the freeway that would be bad it could brake apart and spin free it could warn you by making alot of noise befor it gose so you can stop, these people are idiots they are gambling, no one knows, its happend both ways many times to people who dont have a clue lucky you do they need to fix this NOW!