If it is a shock there easy to replace, 18mm spanner and socket for the top fixing and a 21mm IIRC for the bottom. You need to undo the 13mm bolts each side that hold the handbrake cable bracket to remove the bolts on the bottom. Jacking the trailing arm up 5mm or so helps with getting this bolt out too.
As for replacement shocks......these are excellent....http://www.ebay.co.uk/itm/Peugeot-205-306-309-Gaz-adjustable-rear-shocks-/200819086927?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item2ec1c0164f
As for replacement shocks......these are excellent....http://www.ebay.co.uk/itm/Peugeot-205-306-309-Gaz-adjustable-rear-shocks-/200819086927?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item2ec1c0164f