I agree with the previous poster I would be looking at the shocks and at most of the tire places they will do a shock absorber test for you just to check these things out.

If the results come back as not being anything like as good as new replace them and if you have them tested professionally you will also know if it is both front and rear or just one set or even better if it is just one shock that has gone. If it is the latter you'll have to have a look see to find out why this has happened it could just be a bad shock but it also might be a sign of something else worse wrong with the car.

Col