When I put the truck into reverse and back up, I hear a grinding sound. I can also hear it when I turn fairly sharply to the right going forward. It's coming from the right rear. It's not as loud or noticeable when going forward. It doesn't do it when going straight or turning left. Least I didn't hear it if it is.

A week ago Sunday I could hear a grinding noise coming from that same area when I would brake. I took it to a shop and they told me the brake shoe had slipped off a bit. Not enough to cause a problem, least in the short term, but enough to make a noise. They reset the shoe, problem went away. The sound is similar, the only difference now is that it doesn't do it when I brake. I'm taking it back to that brake shop in the morning and let them look at it. They did tell me there was some piece in the brakes that was, basically, worn out. I don't remember what it was called, they showed me a picture of what it is.

Could it be something else? I'm worried it's the rear diff, again. I've already replaced two of them.

 

If you have replaced the rear differential twice, then something else is wrong with the truck to cause them to go bad. The differentials in these trucks are pretty solid unless they are abused.

 

^^I agree. It would be interesting to hear what happened to cause you to go through 2 differentials.

 

Lots of stopping and going. Like 500+ times a day. I'm a rural carrier. The last rear end made it about 45k miles. They even put in the heavier gear lube to keep it from going out again. Which didn't work obviously.

It was the brakes again. There was a piece in the rear brake that keeps the shoe adjusted, or in place. I've forgotten exactly what they said. They showed me the part they had replaced. It had become worn down over time, that was letting the shoe become loose and rub on the inside of the drum.