This is bad (drained diff fluid)

How many miles on those rears and was the oil original.

I would not be too freaked out by that.

There' is always that mud looking crap on the plugs if the oil has got lots
miles.

As long as there is no chunks or hard slivers run it. Anti seize looking mud is completely normal.

 

What swann said. That's just normal wear i would expect out of a half million miles.

 

Not so fast. Unless you have hard parts on it. The shavings can be normal wear, and long time since its been changed. When you have a thrust washer with a eaton part number on it, or a gear tooth or a bearing cage, you have a problem. When you pm your truck and check boxes wipe the plug off everytime. This way you know if its failing fast. Put new oil in it and run it. Pull axle shafts and drain hubs while your at it. Now as you run it watch for leaking seals. A seal happens, but multiple seals or a seal right after changing you have a problem. Check input, and out put shaft movement up and down. All so allways check to make sure the vent is open. I have see worse run a long time. A past owner may have never wipe the normal wear shavings off and it built up over time.

 

To add to the above, pull the vents and clean them out. Just soak in diesel fuel or parts cleaner while you are doing the rest then blow it out. Make sure its not plugged up. Literally put your lips on it and blow.

 

We have much worse coming out our bodies at that many miles, its when chunks start showing you worry.

 

Normal wear. Fill and ride !!