We do have some different terms.
However a "pumpkin" is a common term here for the middle section of a rear differential.
Also known as a center section, or "third member".
I am sure that SOMEONE can make a joke out of that...
Pumpkinhead, I believe is a scary movie title...
But I degress.
If you are replacing the rear ring (gear)and pinion (gear)in an existing rear differential housing I would replace the side/axle seals. and the rear diff cover gasket. Usually light metal.
If you are replacing the pinion gear in an existing housing the "crush sleeve"is mandatory to get the correct pre-load on the pinion shaft and bearings.
Since you have to remove the rear diff to accomplish this, new rear diff mount bushings are a good idea.
Four new driveshaft to rear diff flange bolts, nuts, and lock washers are cheap insurance as well.
Stock Subaru rear differentials USUALLY last a very long time, what went bad on yours.
Tom