Drain it
****Refill the Rear with a Synthetic
https://www.redlineoil.com/75w90-gl-5-gear-oil.

you may be surprised.

Redline MTL (manual transmission lube) has succesfully quieted and lengthened the service of Volvo M47 5 speed trannies. As discussed at length many times here on the BrkBrd.

You can go with a higher viscosity. What have you got to lose, besides the money for the Gear oil....

https://www.redlineoil.com/gear-lubricants.

____________________________
"""I am aware that differentials in general are overbuilt, and many howl, yet will function for thousands, or tens of thousands of miles, but I still make long trips in the wagon and do not want to be stranded, or fear being stranded.""""

I don't believe there has been any documented/reported Rear End catastrophic failures here on the Brickboard----I've been here since the late 90's.
If there were, responders would concur that you are right to be fearful---but that is not the case.


My 75 245 developed a slight rumble at around 85K in the mid 1980s.
I had to take it off the road in 1997 because of severe body rust.
It was my kayak hauler---to and from the WV mts, to the Carolina mts, Tenn. A couple of trips to Colorado fully racked with 2 passengers and a ton of camping gear-non stop to Denver switching drivers. One trip totaled 5000m -- driving around the state New Mexico--Four Corners. They got some big hills out there in them mountains, not the mere bumps that are the Appalachians.


Also own a 1980 244, and a 1987 245.---the 75 I got in 1983 ---- so almost 40 years driving 240s