The driveway sounds fine if it is flat or nearly so side to side and end to end. The supports I made would take up the same or less space than blocking in other forms and aren't a major job to make.
If you are trying to just drop the keel it seems that I've seen people do that with the trailer under the boat to some degree. Hopefully someone will post pictures or a link to pictures doing that.
You really don't need a bunch of jacks to get the boats off the trailer. Drop the tongue all the way to the ground. That raises the back of the boat. Block the stern in that position stern so that it can't go back down. Raise the tongue. I'd go part way with the tongue jack and then further up with a floor jack. I would recommend one larger than those small $30 ones. Harbor Freight has some for....
http://www.harborfreight.com/3-ton-super-heavy-duty-floor-jack-34271.html
...for $80.00 and I've seen them for less and I have one. You will use it with the keel also. The reason for the jack is that you possibly could damage the tongue jack as raising the front of the trailer high enough to get the boat up and level might put too much of a strain on it. You could also use a bottle jack for this.
Once you have the boat up level with the ground block the front of the boat. Now lower the trailer and remove the wheels/tires and drag it out. This is where you will have to reposition the front blocks a couple times to get the axle past them. If your springs are shot (rusted and sagging) this might be a good time to replace them. They are cheap. Drop the springs and remove the axle and now you can just pull the trailer straight out. That is the second way.
The key to this is that the back needs to be supported very well and while you are moving the front blocks it has to keep the boat from rolling over. I hate to give info on this as you are dealing with a load that can kill you and you need to be very careful, but hundreds of people have done this.
As I said I think you can block only the back of the boat and then drop the trailer enough to remove or at least lower the keel,
Sum
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