You need to replace it. It's not safe. Since you have AWD, you will need to replace at least two tires on the same axle (or find same tread depth, size, maker used tire on Ebay). Then ask a tire shop to put new tires in the rear (if used same depth tire then you don't need to put in rear). If you find a used tire with same tread depth, ask the seller for tire's manufacturing date. The date has to be within 4 years.