Wait....you feed them corn and they get diarrhea? Is this the only time? How long does it last? You may be looking at aflatoxin poisoning and not Marek's. Symptoms can be the same. Stop giving them the corn you now have. Assume it's that and start them on vitamins. Apples, broccoli can be given and will help purge and soak up any toxins in the system. This would also explain why the Duramycin helped them get better.
http://agr.wa.gov/FoodAnimal/AnimalHealth/statevets.aspx use this link and find your State Vet and contact their office to see if this is an issue in your area. Save the bag of corn and see what it would cost to have it analyzed. Here in Maine it's sent out so it runs around 35.00. It may be cheaper for you. If your birds start showing improvement after taking the corn away and adding in vitamins, apples and broccoli, I would seriously think about having it tested. If you still have the bag it came in the lot number and date of production is printed on the bottom where it is sewn together. You need these.
I have to do chores now, but afterwards I will get you some info on the aflatoxins.
Ah, I had missed the corn comments. I am so thankful you were more observant than I was Haunted55!! Yes, this is very suspicious too! I would absolutely stop feeding that corn. I've also read broccoli is very beneficial, along with very dark leafy greens if you have them such as kale. Kale is often recommended for parrots suffering from aflatoxin poisoning so I imagine it would work well for chickens as well.