Are there any all-wheel-drive experts out there who can enlighten me on
something?  Here's my situation:
1) The front two tires on my AWD Honda Element need to be replaced.
2)  The rear tires seem barely worn.
3)   I'm thinking of replacing the front tires only, with the exact same
tire that is currently on the rear.
4)   I've been told by a tire dealer that if there is too much wear on
the rear tires that I risk damaging the drive train.  (The transmission
is manual--if that matters)
Is there truth to what the tire dealer is telling me?   If so, how much
wear on the back tires is too much?   I'm wondering if  the dealer just
wants to sell me 4 tires instead of 2?

As always, any comments on the matter will be greatly appreciated.
Lee Kouns
'88 Westy
'04  Honda Element