If I uncheck 'Scan web (HTTP) traffic' and leave the 'Web Shield' running, will it block urls only?
Well the blocking of URLs by the web shield is only for URLs that you specify. What that option is doing is stopping all scanning for http traffic. However, web shield site blocking, redirects to a local page and is displayed in the browser by the web shield, which should work as you haven't disabled the web shield (and its http redirect, it just isn't scanning http traffic). But I haven't tested that.
So not really sure what you mean by blocking URLs, if that is blocking malicious content in URLs then that would be no.
The network shield blocks known malicious sites.