If he ramp is steep, hook up a tow line to your front bumper before you back down, or at least get it out and have it handy. there are usually plenty of big pickups or SUV's near a ramp, who can help you get started up ot.
In MA we have 10-11' tidal swings, and in parts of ME it's even more, so the ramps are steep, and sometimes wet and/or covered with algae. I've seen plenty of 2WD vehicles that needed help to get up the ramp.
Don't forget to set the parking brake when you back down, or even better, set wheel chocks....A couple of seasons ago a guy in Marblehead sank a new Grand Cherokee up to the roof.