Not all of them.
No one could call Joseph Sobran a Marxist. He used to write for National Review. He died in 2010.
Here’s his book: http://www.amazon.com/Alias-Shakespeare-Joseph-Sobran/dp/0684826585
http://www.sobran.com/oxfordlibrary.shtml
Cindy Sheehan’s wife? He may be a Marxist.
Good refutation of that book is “Contested Will” by James Shapiro. One thing that now appears clear is that he did collaborate with others on some of his plays and that some of the plays in the early print editions had been butchered from his original manuscript versions.