Are the wheels just bent and do they have any play ? , jack the car up at the back so that both rear wheels are off the ground. check for play / wobble e.t.c. It may be the trailing arm bearings that are shagged these are replaceable with a new bearing kit, about £50 notes each side or it could be the axle. If it is the axle some guys from this website gave me this web address
http://www.imaxle.co.uk/ they provide pug / saxo reconditioned rear axles. For a reconditioned rear axle they quoted me £354.00 all in, this included fitting and re adjusting the ride height, seemed quite a good price to me. Hope this helps.