I fixed it.
The issue wasn't the content type. I was using "application/json" for what I posted originally, and when I tried your suggestion of "application/x-www-form-urlencoded; charset=UTF-8" the response that I got back was "error":"invalid_request","error_description":"The \"grant_type\" parameter is required."
At any rate, the issue ended up being me using Eloqua's Templated URLs for the call back url. Once I removed those it worked fine.
thanks for trying to help,
Chris