Manly has definitely been slippery for us too.
I have a 4x4 BT50 that has started sliding backwards after driving the boat onto the trailer (with a boat catcher).
Bit of power on the boat engine to stop the slide, climb out to the ute, engage 4x4, and drive forward 1m, go back to boat, power down, tilt up, back to ute and drive out.
It was a bit of stuffing around, but, the feeling of the whole lot sliding backwards was very uncomfortable !!
I am a fan of having it in 4x4 now and use it always (because it's already there). I think a diff lock may not help that much if the ramp is slimy across the whole width.
Someone mentioned sand or similar, I like that idea too, you'd only need it if the ramp was slippery at low tide.
If it's possible to avoid dead-low tide on a slippery ramp that might help too.
It may not be needed to spend extra on a locker/4x4 with a bit of planning/scheming