I'm not sure if my opto idea would work,the PIC might not be able to source enough current to turn the LED part on.
An alternative would be a couple of BC108s or similar connected as a Darlington pair with the second one operating the relay.
The input impedance of a Darlington is high,as the first section is really an emitter follower.You can put a fairly high value resistor(I used 82k Ohm ) between the PIC output & the base of the first transistor to reduce loading even further.
If you use a real relay,all the problems about earth level shifting,etc go away.
Grab a few components & try a few of everybody's suggested configurations,or model them,it's a lot quicker than asking people on a forum.
VK6ZGO