Besides galaxy recession/approach, what internal galactic process can cause a differential Doppler shift?

Answer

Rotation within a single galaxy.

Differential Doppler shift occurs because one side of a rotating galaxy moves toward the observer (blueshifted) while the other side moves away (redshifted).

Besides galaxy recession/approach, what internal galactic process can cause a differential Doppler shift?
astronomyuniverseredshiftDoppler effectblue shift