Replaced the tailgate struts on my 1990 Wagon. The struts screw into the ball joint ends attached at the car. On mine the shafts where they screw into the ball joints were rusted and frozen.

I propped up the tailgate in the upright position overnight, applied Kroil down into the threaded sockets where strut shafts screw into the ball joints. This morning I took off the clips where the strut connects to tailgate, and one strut unscrewed off of the balljoint just by looking at it. The other only took slight grip and turn with a grip pliers.

Don't underestimate Kroil, just have to give it chance to do its thing.