If this happens, in many cases it is not enough to replace the shimming ring. The differential seal is worn. The differential half-shaft bearing is worn. it is advisable to replace the bearing and the seal on both sides. The two sides are not the same! The condition of the bearing can be easily checked. If you move the half-shaft perpendicular to the axial direction and it rattles, the damage to the seal is caused by wear of the bearing.
You need the part number 3, 4, 5, 6. Check the exact data in fiat ePER. You have to do this because there are so many variations. But if you enter your Fiat car code, you can order what you need.
These parts are sometimes hard to come by. But I usually succeed from here. According to my experience, it is a very fair company. Fast, cheap, precise.