Sarah, on her website, tells the "secret" and limitations of dehydrating cooked chicken.
Any *dense* *large* piece of meat is going to be terrible to try to dehydrate. That is one of the reasons that beef jerky is often in thin strips.... avoid thickness - avoid the *Large* issue complicating drying.
What can you do about density? Wet cooking such as boiling, softens up the meat and throws a bunch of water into it. Broiling dries out the meat, some, and closes up any "drainage" channels it may have possessed.
Sarah reports that cans of precooked moist chicken dehydrate pretty well, especially if the pieces are broken up into small pieces.
To the eye, the appearance of precooked canned chicken meat suggests a looser fiber structure than broiled meat. Less dense or more permeable might be a better phrase.
Sarah's site is great for solving problems. I'd go read what she has to say about the spoilage free life of home dehydrated chicken.
Once in a while, at GoodWill, you can find old books on dehydrating food to use as a guide. They are often less than $1.