Seperating driftwood from stone, sealed with silcone

You might be able to just physically rip them apart. I don't think silicone bonds terribly well to wood, and it doesn't bond real well to all rocks either. If it doesn't work, I've heard of people using wires and guitar strings to help cut through silicone when derimming a tank, so that might work.

What do you plan on doing with it afterwards?

I've had really good results bolting driftwood onto slate, but that has to go in before the substrate.

I am hoping that is will sink on its own now. I have about 10 anubias petite attached to the wood that are doing well so I am going to put it back in the tank in the same spot.

Guitar string - genius

You may not have any trouble at all. When wood gets wet it swells slightly and begins to get a tiny bit soft. Since silicone doesn't soak into wood but just sticks to the surface, it may pull off real easy.