Body Flex
 

I have noticed a odd happening while driving on wavy roads. My steering wheel will turn and roll with the pavement. It feels like body flex. Also, I get a mean shimmy at 75+ MPH.

What can I do cheaply to take care of the chassis?

I believe I need the front end checked for the shimmy. But I want to know if both symptoms would be caused by bad ball joints? And wouldn't that show up at all speeds?

Thank you for any help!!