Based on the rules of your league I can see why you would dump both of them.
Dunn - Is not gonna produce much this year. He's out of shape and didn't commit to an offseason program, costing him at 32.
Rios - Has been unlucky and is a streaky hitter to begin with. He seems to be showing signs of life and should begin to produce for the remainder of the year. At least I hope as an owner.
I would definitely dump Dunn as he's not performing this year and will not be worth the price of admission next year. Rios on the other hand will produce something this year, but probably won't be worth the price of admission next year. So I would definitely try and move them. Zimmerman and Brown would be favored over the Scherzer/Brown option.