My recommendation after working several years next to a busy ramp:
Get the truck and trailer lined up with the ramp and put in 4H or 4L (preferred) prior to decending down the ramp. With the truck in 4H or 4L, the ebrake and transmission are engaging front and rear tires helping prevent the truck going into the drink if the rear tires are on a slippery surface or if the load on the rear tires is reduced during loading. I've had too many close calls with a 2wd truck and seen too many trucks go in the drink to not use 4wd when it's just a push of a button.

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