I am renting 2 U-pack units and picking it up tomorrow, Tuesday. How are the units placed on the trailer, in other words can I access the doors on each u pack so I'm able to pack units while the are on the trailer; r sides trailer removable?

The door on the back of trailer can be accessed there is a ramp that comes down by pulling it up to give you access. You can ask the representatives ar uhaul to show you how to get ramp down.

They are able to be loaded on the trailer i believe. Each U box has a door on the front that faces toward the back of the trailer

I have never used one !