Quote Originally Posted by Fivefingerdeathpunch View Post
You still have to replace the whole axle, if it lost grease or the parts inside failed, putting a new boot with grease on won't fix it.

I would also double check and make sure your motor mounts and tie rods are in good shape, you don't want any play there.
^^ this. I had the same problem but mine would do it at 30mph+ I had to replace the whole axle and tie rods.