Sunset SS

My 01 SS has had a clunk or rattle in it since new. I notice it when I ease through a rough parking lot or rough or uneven ground. I recently grabbed the rear wheel and shook it and heard the noise. I checked the lug nuts..they were tight. I then removed the wheel and noticed that I have a tiny bit of in-out axle play. The brake pads and or calipers is where most of the clunking is coming from though.

Is this normal? How can I stop it?
Thanks,
Eddie