Might not be anything to worry about.

The brakes are set to work in a praportional manor the #s are usualy around 70front/30rear or 60fr/40r and if you were going slow on ice in your drive way and applied them just a bit the front would be first to lock up and the rears would not lock up until you gave it a lot more pedal presure.

I would give you some exciting ways to test that theory, but I sugest you just 1.Jack up the back of the car and put jackstands under it. 2 Block front wheels. 3.Put it in gear and spin wheels at about 20 mph. and apply the brakes and see if they stop the wheels.

If they work make sure wheels are stopped before you put the trans. back into park.

If they don't work shut the eng. off to stop the wheels before you put the trans back into park.

If you cant get the rears to stop doing this then I would start to worry.