Don't worry about it man its called limited slip differential its for cornering to transfer power and yes they should spin opposite unless they are under load. If you want your dif to be posi you need to tighten the nut usually inside the rear hub carriers. I dont suggest you do that it will work your dif hard and well im sure your not just racing in a straight line where posi would help the most. Im pretty sure my info is correct..

O.K.