I used chain link fence, a big tree, and some chain to unload my trailer and it worked well. But I was unloading horse poo so it stuck together a little better.
I got the idea from another thread somewhere on TBN and it worked well.
Basically, get a length of chainlink fence that is twice the length of your trailer, lay it flat on the trailer with half of it rolled up at the tongue end. Load your rock on top of the chain link fence, back up to a tree, unroll the rest of the fence over the top of the rock and then devise some method to tie the ends of the fence to the chain and wrap the chain around the tree. I used some steel pipe and a fence puller. Then drive the trailer away and everything should slide off the trailer. If you have good traction you can grab one end of the fence and pull it out from the rock pile or just unload the rocks until you can just lift the fence out from the remains.
Did this make sense? Let me know if you need more description. Its another option to consider.