Mub Flap recommendations for a 2017 Ford Dually please.

There are a lot of various types of mud flaps out there. But if you want to keep the "junk" off your 5er the flap will probably need to be about less than an inch off the road.
Measure where the water, dirt, mud & junk will go, use a string under the tire & lift the string to where you want the flap to cover and it will be close to the road.