The obvious question... What's the IO pin voltage measuring with no relay connected? (if it's 3V, yet you're only getting 1.8V when driving the relay, then it's your relay pulling too much current for one pin - try paralleling two pins & having them both drive the relay coil....if that doesn't work, then it's your power supply feeding the PIC (incidentally, is the PIC power supply pin holding up at 3V when connected to the relay coil?). The normal method is to drive a transistor from the IO pin which in turn drives the coil.