Saw this a little post a just now after having very similar issues.. at highway Speed and at slower speed between 30 and 40 mph. Only When I let off the gas, in between accel/decel[ coasting.] I would get a huge grinding noise. I thought it was the rear diff or transfer case gears grinding... was very loud and horrible sounding... I removed the rear drive line and drove on the front with the center diff locked Thinking that would solve the problem. But I still had it now coming more from the front. After racking my brain for hours I finally disconnected my front hubs, Yes I have AVM hubs, And ran the truck without the hub's locked, stationary, so that the transfer case and front diff was turning... At first, there was no noise until I gave it a little gas and then let off. And then the horrible Grinding/ clanking noise appeared steady. It was the drive shafts. My drive shafts were installed By someone else, out of phase, where they you joints were not lined up... Also U joints appear to be loose though not bad and probably needed greece... I'm off to get the drive shafts balanced and greased up To solve the problem.. Here is a video: