Without more information, my guess would be something related to the rear differential. When was the last time the fluid level was checked or fluid replaced?
I don't own an AWD, so I don't have any first hand experience, but I've heard that the rear diff can be an issue on the AWD cars. Hopefully someone a bit more knowledgeable can chime in.
2005 Vibe GT ~ Platinum
2006 Vibe ~ Lava, Base, Auto
2005 Corolla S ~ Silver Streak Mica, Manual (Wife's)
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete fools.