Choices:
Buy New Axles made in China or some other place and pay $100-$150, but no core charge and no shipping to pay to ship the cores back.

There are several large threads on Axles and few Members have had trouble fitting the spacers onto the new China Axels (be sure to save the spacers what ever you do). One or 2 have had trouble with the splines on the axles fitting.

Rebuilts from Kragen (got mine for $78 each, rebuilt by Cardone but I believe the price has gone up) or another auto parts place. The advantage here is you get a warranty and someone local to deal with if you have a problem.

Send your Axles to be rebuilt by a High End rebuilder. Cost close to $300 each and you pay shipping both ways; have a waiting period in which your car will be down.
Used Axles from the Junk Yard. Main advantage is Cost. It is more labor intensive as your will be doing 1 1/2 axle jobs; more if your used Axle turns out to be bad.

When buying a used Axle from the J Yard it could pay you to buy that insurance on the part so you could return it is bad from the start. Won't help if it goes bad several months later.
If your Axles are not making noise and only the Rubber Boots are damaged you could just replace the lube and Boots (again extensive threads on this).

Best to spend time reading the Threads.

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84 300D, 82 Volvo 244Gl Diesel