How am I going to re-attach this jack?

Of course you already know that will all have to come out and be completely rebuilt. Elkhorn's are noted for their water intrusion. It doesn't look like there is much to attach it to is my point. I see some dark wood, rot maybe? It looks like the mounting plate came of with the Jack the bolts are still mounted to it so that tells me of rot damage or did you tear it off or did it fall off. I'm guessing by the location on the truck you might have just pushed it off while mounting or did it just come that way? All guesses without many more pics of the damage and some detailed info. Your in the rite place though.
Joe