I disagree with the first line there, Windchill
I've only got stuck on the bottom of a slip once so far, and that was due to it being heavily weeded, so even a four wheel drive would have had issues!! - pretty sharp inclination slip that our loadall at work will happily slide on too
When launching, the boat is very keen to leave the trailer even on a shallow slip. I normally back the van down the slip till the back tyres are just out of the water - I then tip toe around the back of the van without getting wet, and climb over the bow onto the boat. Drop engine onto shallow mode, and bung into idle ahead - reach over the bow and unhook. Drop engine into neutral, and the rib slides nicely into the water every time.
Once on Neyland slip, I undid everything, went into neutral and the boat didn't move, so I thought I'd get off and have a look - just as I jumped off, the boat launched..... it was a very quick jump back on board!!
As for picking it back up again, I do the same thing mostly in reverse..... on the slipways at Milford and Neyland, they're concrete, with very little/no stones, so I am happy to use a bit of power and drive the boat onto the trailer. If there is weed/debris visible, I normally jump into drysuit, overboard, and winch the boat onto the trailer as opposed to driving it on - I like the ali prop with its black paint on it
When going up the slip, I generally take bite with the clutch, and let it slowly pull up not riding the clutch too much, and then engage clutch fully as soon as there is enough power in the engine to pull it up straight. Sometimes there is a bit of loss in traction, but careful control beats that mostly, and it pulls the boat up everytime. I'd guess the setup is around 900kg all in - my previous van didn't weigh too much more than that I don't think, but the manual reckoned it was good for a ton in pulling
Although I'd love a 4wd for weedy slips, we just don't have many like that around here, so 2wd is just fine