The shop I went to couldn't lengthen it because of the round joint...wouldn't bolt up to any of his fittings. When we measured the shaft, it seemed long enough anyways. There is no binding, so I don't really know why Superlift says you need a new shaft.
By the way, I know that disabling the AWD will still allow the shaft to spin, but it will relieve some stress on it and it is a good mod for other reasons as well.