Putting chains or cables on your big 20's is not a good idea.
They probably won't fit.
Check the clearance between the back of the tire and the inside of the wheel well that will be the tighest point.
I've always put chains on the front of four wheel drive pickups, if chaining one axle. If you do end up with chains (or cables) I would recommend doing the front. You will maintain better braking and steering.
But again, if you are just driving around town, chains or cables should not be necessary at all. Just slow down.