It's not a big deal to dunk the boat in real deep to get it off the trailer. Why not remove that entire swivel assembly and put a $10 roller in its place? Even as it sits now, if you want to launch it, get the trailer deep into the water, put the rear wheels of your truck in water if you have to. If you have any trouble coming out put it in 4x4 low range and just make your way up the ramp.
My boat is heavy and sits on bunks, I pretty much have to dunk the trailer all the way to launch it.
Pro-tip: Disconnect your trailer lights when you dunk the trailer and don't reconnect it until the lights have no water in them. The 'submersible' lights most trailers have, like mine, fill with water just as quick as any other. If you don't disconnect it, you will be replacing bulbs every time you use the thing.