Well, it's kennel cough..
Just got back from the vet with the diagnosis. Poor Diesel coughed all night last night. I took him in this afternoon and he was given a shot of antibiotics and a shot of antihistamine in the office, plus some more antibiotics and antihistamine in tablet form to take home for the next week or two. He also brought home some cough syrup. Rocco is starting to have symptoms so I am going to keep an eye on him and if he gets any worse I will take him in too. All is good right now. Diesel seems to have more energy today and seems in pretty good spirits, so I'm sure he'll be okay.