if its too hard to lift the boat in a sling (most trailer shops could help you out there if you need help) you can lower the bunks below the level you would need them to be (the boat can then rock from side to side), put your boat onto the trailer at a ramp and then jack the bunks back up after you pull clear of the ramp. to make it easier to get the boat on and off, you dont need to jamb the bunks up under the boat - the boat should be able to rock slightly from side to side on the trailer.

Andrew