Lots of penetrating oil to start. Soak it good. The Vacuum actuator motor, that is what the plunger is called, has a brass fitting that slides into an ear in the axle tube. It is most likely seized. Pull the boot out of the way the and release the e-clip. Try to pull the actuator out of the ear. Here is a picture of what happens if you hit the ear with a hammer.
I should have used more penetrating oil and less hammer.
The penetrating oil might be enough to free up the rod in the actuator so give that a shot first.
.
.