I just picked up a nice '84 300CD and the trans needs work but I do have a good trans in my '80 300SD and would like to know if I can use it in the CD?

First check the speedometer connection (electric vs mechanical).
If they're different, most likely not. -CTH

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the 80SD will have the 722.1xx while your 84 has a 722.3xx
Different transmission and the earlier one is not as beefy. You could make it work although there are differences to overcome.
If you are capable of swapping tranny's you are capable of rebuilding one, the friction and seal kit is only $200 for them. They are really pretty simple.
Whats wrong with your current trans?

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81 300SD daily driver/project
86 420SEL (sold)
85 380SE(in the graveyard)