2001 Chev HD2500, 265K miles, (Eaton) 11.5" ring gear:
Something I've noticed for quite some time is that after traveling a long distance (100+ miles), I feel and hear an audible "clunk" after moving in reverse- meaning, if I put the truck into reverse and back up a few yards, eventually, I'll hear/feel this noise. I run the Mobil 1 70W-90 synthetic lube and have been for years. Rarely do I carry heavy loads and my towing usually consists of a john boat for duck hunting or an ATV/trailer. I travel 250+ miles one way 2X weekly for work. When I've pulled the cover and changed lube in the past, I've never found any debris in the lube and there are no teeth missing that I can see from the R/P or the side/spider gears. Is there a possibility that the clutch disks are binding and finally "let loose" when backing up? I noticed this condition long ago (before I had 50K miles on the truck). I hear no bearing noise coming from the rear end or from the wheels.

Any thoughts?