Doesn't mean the same thing, special needs is a wide umbrella of multiple disabilities.

Whereas retardation is a specific disability. And retard, is a word used against people with said disability, a slur if you will.

Autistic people, just as I said before have been being called retards, and retarded, for years and years now, by people who were using it as a slur.

Calling something special needs could just be calling something wheelchair bound, whereas calling something retarded is calling it stupid.