If you are only planning on replacing the keel cable, you can do that while the boat is lifted, but still resting on the trailer.
Use a couple of straps to strap the trailer axle to leaf-springs. You don't want the trailer to lift as you begin to unweight it. Next, nower the tongue jack as far as possible. Take a couple of saw-horses and put them under the aft end of the boat. Place a couple of 4x4's across the saw-horses and build up the wood until it comes in contact with the stern of the boat. Then raise the tongue jack as far as possible. This will begin to lift the boat off the trailer bunks. Take some scrap 2x4's and fill in the space between the bunks and the boat to add some stability. Continue repeating this process until you can lower the keel enough to get to the keel cable bolt. Once you have the boat lifted enough off the trailer, then support with some jack stands. I have done this several times now, unfortunately.....
Kevin