i bought one of these a while ago,take the n/s driveshaft out(it should be 3 bolt type at box)

when you get the shaft out in the box there is a circlip that holds the diff in place,more than likely it will be broken,they are about £1.80 from renault

easiest way to tell is to get in the car and drive it,if the speedo goes up but the car dont move it will be this clip

hope this helps mate