Hey, nevermind. The correct answer to my own question seems to be, "pry the socket off as far as possible, then smack the strut with a rubber mallet." The small hole in the socket is actually a pair of holes that the retaining clip slides through. Nevertheless there was no clip on either of my stock Volvo struts.

Other considerations -- as you might expect, the tailgate WILL swing down when you remove one of the weakened struts, so either have all parts of your body out of the way, or do as I did and swing your legs up to catch it. (Pointing this out probably says more about me than anyone else, but hey, whatever I can do to keep Darwin at bay. :^p ) Now I'm just waiting on a buddy to hold the hatch all the way up while I smack the strut socket into the ball on the hatch, and I'll be set.