85 300CD Rear Axle Cost of Replacement

A Mechanic wants $1000 to install new axles in my 1985 300CD. This includes parts and labor. Is this reasonable? I have a 1979 or 1980 300CD parts car. Are these the same alxes? How difficult to do do the job myself? Can I use these axles? Are the 123's axles all interchangeable for these years?

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