The shaking began after the washer was a little over two years old and seemed to occur on high spin speeds, so I used medium. After a couple of months, the washer would shake violently no mater what spin speed was used and result in the dc unbalance code.
An appliance repairman told me replacing the damper rod assemblies has fixed this problem every time and would cost me $250 for him to replace. Even though these dampers looked good, I followed his advice and replaced all four Damper rod and spring assemblies myself for less than $60 with parts from Appliance Parts Pros. He was right and my problem has been resolved.