HTTP Request - Setting HTTP Request Body in Groovy

I'm try to change the body of an HTTP request before I make it in groovy. The goal is that I keep making the http request using different values (pulled from a data spreatsheet) in the body to get different results. 

I've tried a number of things related to:

testRunner.testCase.testSteps["HTTP Request"].testRequest.response.responseBody    (Just an example of an attempt)

But have had no luck so far either getting the body or setting the body. I did have success setting the headers before hand. The body is just giving me trouble.

Thanks in advance!