I just reaquired my 84 300D Turbo (my son had it for ten years), and whenever I make a left hand turn, there is a loud thumping noise coming from the right rear of the car. A "shade tree mechanic" neighbor says I need to replace the diferental. What are your thoughts?

It could be several things (do a search for rear thumping sound).
Since I have been a member of this forum most of what I have read concerning rear problems have been CV axles or wheel bearings. With a nod towards the CV axle as causing all kinds of sounds.
You did not say if the "ST Mechanic" jacked up the car and actually looked under the car or not.
There could be some other suspension problems going on (a loose shock will also thump).
It is also not to difficult to take a peek inside of the rear differential and you can change the oil there while you are at it.

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84 300D, 82 Volvo 244Gl Diesel

You may want to check your sway bar links first, easy visual.
This is on my SD, but the 123's have the same set up. You may need to jack up the rear, use a floor jack on the diff, if you can.
Welcome, much info here.

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83 SD

84 CD

Thanks for your advice! I'll have a qualified mechanic check the CV axles and sway bar links.
gdl40