My mechanic says my noisy car is telling me the differential is going. Shouldn’t I fix this before it goes? Is it worth it at 100,000 plus miles?
Unless you trust your mechanic explicitly, get a second opinion.
Differentials are not cheap to fix.
To answer your question: Yes, fix it now before it leaves you stranded somewhere.
The second part of your question addresses whether or not you still want to keep this vehicle.
A private buyer may or may not want to buy it in it’s present condition.
A dealer will no doubt give you a lot less in anticipation of a big repair bill.