Puppies.. and even adult dogs shake things naturally at play time
Titan did as a pup but no longer as an adult. He would grab a toy and shake it back and forth really quickly.. totally normal.
With biting your hands, feet, etc.. that's him trying to play. Say "no" or your command to stop, and replace with toy he can thrash around a bit
Commander Charly Titan (GSD)-15 Jan 2010