You can't replace only two tires.
"... all-wheel drive vehicles are equipped with additional differentials and/or viscous couplings that are designed to allow momentary differences in wheel speeds when the vehicle turns a corner or temporarily spins a tire. However, if the differentials or viscous couplings are forced to operate 100% of the time because of mismatched tires, they will experience excessive heat and unwarranted wear until they fail."
Tire Tech Information - Matching Tires on Four-Wheel Drive And All-Wheel Drive Vehicles
Start by replacing all four tires.
