We did not use the ramp the Uhaul utility trailer came with, it is far too steep and made of stretched steel which will grab the casters. Also the trailer bed is ribbed which will mess with the casters also.
We put it down, and I built a wood ramp out of 1/2" plywood and 2"x 4"s as ribs.
2 sheets 1/2" plywood 4" x 8'- 1 for ramp, 1 for bed of trailer; both full sheets, uncut.
3 - 2"x 4" , used for runners, cut at angles to lay against the angle of the ramp and ground.
2 - 2" x 4" , used for ribs, cut to size - this is imperative to add strength to the ramp.
see first pic, that is the bottom of the ramp.