I have a tub faucet with a built-in shower diverter that's leaking. I've read the thread about removing the entire faucet and taking it in to buy a replacement, but my problem is that I can't get it off! There's no screw holding it in place, but when I try to twist it off, it's stuck fast, as if it's glued. I'm worried about tearing the entire pipe out of the wall. Is there anything I can use to loosen the thread?