That's what I do when I am unloading my utility trailer if it has been unhitched from the tow vehicle. Just a block of wood under the back end is all it takes, simple and effective.
I'd never load or unload a tractor onto a trailer not being connected to the tow vehicle
,......... especially if he's already stating the weight of the tractor is lifting the rear of his truck being connected to the trailer as it is,
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Do you have hinged ramps? if so you should have some braces weld onto them that would extent to the ground when the ramps were let down, if no hinged ramps you can do as others have suggest a couple of stacked 6x6 blocks or what ever size would take to sure up the rear of the trailers frame, another option would to have some hinged ramps with down braces installed, would sure make life much easier,
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You could also have some square tubbing approx 10" long weld to the sides of the back end of the trailer and use the next size smaller tubbing to fit just inside the larger one have sets of holes drill into the smaller tube that a Pin can be put into when adjusted down to the ground,
kind of on the principle of jack stands for supporting cars etc, ....
in fact might could use a pair of trailer tongue jacks that are built on swivels and locking pins, one bolted/welded on each rear side corner of the trailer,
Here is a quick sketch of what I'm trying to describe