I had my rear diff flushed and the mechanic showed me the fluid which looked very metallic/ sparkly. He said this is not good and that the differential will eventually need replacing. He said don't worry about it till it starts making noise. I'm about to do a long road trip with a trailer. How likely is the differential to fail and leave me stranded?

 

No way to tell really. How long was the last oil in service??..... How great, or small, is the pinion backlash at the input flange?? Any in/out or side to side play there?

If it's not making noise, it might last a long time..........or a bearing might get hot and it will grenade.

Based on my own assessment of the things above, I'd either run it, or not. How lucky do you feel??