2003 Grand Caravan AWD - AWD not working.

My 03 Grand Caravan AWD has a problem with the AWD. The rear wheels never kick in -this worked fine last winter - I just noticed the
problem because of the show we just got - the problem may have been around for quite a while without my noticing it. It essentially acts like a front wheel drive van now - terrible in the snow. Any advice would be appreciated. It still has the 7/70K powertrain warranty in effect - Hopefully it would be covered if I have to bring it in.
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