Radical and fundamentalist have similar connotations to me: modern strict interpretations of old religions under the guise of faithfulness to tradition. A difference between the two might be that radical groups tend to challenge the stability of society whereas fundamentalists seek to live quietly within society.
Fundamentalism in a Christian context also has a particular meaning in relation to the American evangelical churches who signed up to the five "fundamentals" of the Christian religion in the early 20th century.
It's an interesting and open question whether JWs are either fundamentalist or radical. I don't think they qualify as radical, but possibly as fundamentalist.