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