How to add rear rack to a full suspension bike? I'd like to carry 20 - 40 pounds or so in panniers on my rear rack.
I've carried that much on my hard tail so it's not a problem for the biking itself.

The cantilevered rack style that is supported only on the seatpost is a non starter. I have a carbon fiber seat post. It can't hold more than 10 pounds (even that worries me)
I recommend against doing this. You will be dramatically increasing your unsprung mass, which will play hell with your suspension's dynamics. You might want to consider a trailer instead, or see if you can carry the load in a frame bag and/or seat bag.