You will need to replace both of them, if one is clunking don't hear anything from the other its not far behind so do both it helps to tighten up the front end.. if you are going to do the job yourself measure the distance from the center of the back wheel to the center of the front wheel right it down when you replace the bearings you will have to set castor, using the dimention of the center wheels will put back this setting..
Bob