At my hub (Maumee, OH), I was started as a loader, and about 2 months in my PT Sup asked me if I wanted to go to unload because they were short on workers for that day. I tried it out, and needless to say after the first 2 hours in the unload I just wanted to go back to my trailer in the load. And you should be able to switch over to unload so long as certain positions in the load/unload areas are open/filled.
And just a little bit of my own experience during my 3 hours of unload, it was awful. Almost none of the walls were packed tight so when I would pull a package out the wall would just crumble. It is rather nice to work at your own "pace", but I much prefer loading. But who knows, unloading might be better suited for you.