differential/drive shaft
Being your car is a '76 it would only be a 904 if it is a stock automatic.
The top two (221 & 222) are the two Chrysler trannies.
http://www.hurst-shifters.com/catalogpdf/8-Trans-ID-Charts/AutotransID.pdf
Welcome to the Mopar world, I am an old Buick guy myself.
Chuck
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