Merrill,
Chesapeake could be allergic to something in his new diet also. By a process of elimination you'd have to find out what it is. Another Lab that I know was recently found to be sensitive to preservatives in foods- she had bad diarrhea. She is now on a dry kibble with all natural preservatives and is doing much better.
Is Chesapeake a Lab or a Chesapeake Bay Retriever? If he is the latter, they have an oily coat and that could be causing his odor.