My Wife’s Explorer rear diff grinds after long hiway driving. around town it never happens,but on long trips once you start a turn you can hear and feel the grinding coming from the rearend. it is 4wd
I have had a growling differential go another 40,000 miles. If you want a fix, the cheapest is usually a used rear axle assembly. We used to call them “drum to drum” but with rear disc brakes I guess that is out.
Check the air pressure, size, model and year of each of your wife’s rear tires. They should be identical at least in terms of type, size, and inflation.
What year, and how many miles? Try a gear lube change. If that doesn’t help, you probably need to replace the rear axle assembly, like oldtimer11 said.