You can only get some rare items in meelee mode, while others on cooperative mode.

As far as I've played in cooperative, the items you get depend on the average level of your entire party. That's something I've noted.
Last time I played, my party average level was 29, and I got three items on melee: A Coral Sword, a Wizard Robe, and a Lightning bow.
You get random items? I don't know, but maybe there is a pattern to know at least what kind of item you'll get.

I'm on chapter 3, by the way.