UPDATE!
Hi everyone! I just wanted to say thanks for all of the replies to this question/issue, and that we can consider this issue closed!
My apologies, as none the issues listed were the actual culprit or reasons for my crashes. My issue has been resolved and the content.Unload() IS and always has been working in MonoGame!
The issue was the amount of content that I was trying to load between levels causing my app to hang and ultimately exited by the system!
Thanks again!
MONOGAME ROCKS!