Have a 87 k5 blazer 4x4. When I am in reverse and at slow speeds my rear diff makes a horrible clunking noise. It almost sounds if its binding up. I changed the fluid and replaced the cover and nothing seemed loose, no shavings of any sort. It's not lifted. could it be the driveline? If anybody has an idea what might be causing this hit me up thanks.

 

U joints! Even in a stock K5 and driveline angle the rear joints fail reliably! Replace with fully greasable ones...and keep 'em greased! Stick a crow bar in there while its in either park or 1st with the brake set,any slop? Replace 'em!

 

sometimes, worse case scenario , if one of the carrier bearings go bad it can cause this. if you go forward the ring gear will grab the pinion gear and if you go back it might push it out. ive seen this the other way around too.

best bet would be to see if you can recreate it with the rear end off the ground and have someone lay under there and see if they can see any movement or noise coming from anywhere.

might be a broke brake part in the drum also