What the guys with fin keel capri's do is put a heavy duty wheel for a dolly on their trailer.
When they get to the ramp, they, attach winch straps to the trailer and the frame of the truck. When they chock their wheels, then unhitch. The straps keep the boat where it is (this requires a decent tongue weight, otherwise the angle of the ramp could slat the boat rearward on the trailer).
Then slowly unwinching the straps the boat goes into the water.
If the tongue extender really works with your ramp, then just use the straps to keep the boat from "rolling away."
Finally, you have a REALLY big truck right? How's this for overkill?
Can't find a picture of a strap launch, but enough people do it (J-24 comes to mind, as does Capri 25).