I think sometimes the shortening sounds more dated than the 'proper' name. So Viv, Kathy, Gill, Bev, Sue, Mandy would make me think of people about my mum's age, whereas I wouldn't be quite as sure with Vivienne, Katherine, Susan and Amanda.
I don't find Ruth particularly dated, but I'd expect you to be a churchgoer if you're under the age of about 45 and called that!