If your vehicle is showing vehicle rolls when parked despite being in park, here are the most likely causes and the parts you'll need to fix each one.
The linkage or cable connecting your shifter to the transmission has a broken bushing or is out of adjustment. The shifter handle says 'Park', but the transmission is actually physically stuck in Neutral or Reverse.
In many front-wheel-drive and open-differential vehicles, if one CV axle breaks or pops out of the transmission, the differential will spin freely. This completely bypasses the transmission's Park mechanism, allowing the car to roll.
On 4WD or AWD vehicles, if the transfer case is accidentally shifted into Neutral or the electronic shift motor fails between gears, it disconnects the transmission from the axles. The Park gear will not hold the vehicle.
The internal metal pin (parking pawl) that physically locks the transmission gears has snapped. This catastrophic failure is usually caused by slamming the vehicle into Park while it is still rolling.