How I unload logs off my trailer with sides. 

Sling chains under just a few of the logs, not all at once. Or straps. Or continue doing what you are doing, as it works.

I lay a tow strap across the bed of the trailer before I load. It allows me to choke the bottom layer of logs and pull them off the back all at once. But, what do I know...

you need a grapple