HELP!! Clunk noise from driveline.

Last few days I've had a clunk from the driveline. It happens when I first start from a stop just as I let the clutch out and the truck starts rolling. Usually just once, sometimes twice. Also happens while rolling down the road. Doesn't matter what gear it's in, but under acceleration. Sometimes also does it when using the jakes and slowing down. Does it in both low range and high range. I raised one set of wheels off the ground and turned them by hand. They spin freely if you turn them real slow but if you speed it up, they lock up and stop. Does it on front or rear drive. When it does bind and stop, you can see the pinion shaft on the front driver where the driveshaft connects, move in and out(fore and aft). I'm thinking that my power divider is starting to go. I pulled the fill plug on the front driver and the fluid has glitter and its dark colored and the magnet had lots of fuzz on it. Any ideas on what could be going on? How to check it? Thanks in advance.

 

Well it's definitely the power divider. Found a set of diffs with a better gear ratio and only 200k miles on them. Gonna swap them out this week. This is gonna be fun.