It might be the ring gear and pinion. I had another truck that after 200,000+ miles the rear end seal let all the fluid out and cause it to get some shavings. I replaced the gasket and filled it back up and it was fine. It still ran and pulled great. The sound was from the gears being worn down a little. It was okay to drive, just make sure their is fluid inside, just had the loud noise. You can just had some fluid and drive, but you might want to get it fixed. I don't think it will cost a lot to get it fixed, compared to letting it get worse and doing a lot of damage which will cost a new rear end or so.

 

98 Ram 1500 same problem ended up needing a pinion bearing