So I'm just minding my own business letting the tailgate down on my '89 'Cruiser when all of the sudden there's this loud crack/pop/klonk sound and the tailgate instantly weighed, well, whatever it actually weighs instead of how light it felt before the "break."

I am really hoping that this is not like the last time I heard a similar sound when a torsion bar let go in a rusty '57 Plymouth I owned eons ago. Does this likely mean I have to disassemble the whole thing, and if so, what support/spring/whatever broke on me?

Everything's still 100 percent in terms of function, it's just that the thing weighs SO MUCH MORE than I thought it did pre-failure.

Good news or bad, let me know. I'll just have to tell my kids to stay away from the tailgate when I'm opening it to prevent them from being crushed when it drops...

 

There is a torque rod that runs through the bottom length of the tailgate that twists when the gate is opened.
It is retained on the RH side and exits the LH side, by the wiring boot. It attaches to the body by a link next to the wiring boot. See if there is any loose or broken parts there. Let me know what parts are loose.

By your description of the sound, I bet it broke inside the tailgate. Reminds me of when one of my power r\window regs broke...

The instructions for replacing parts are in the Fisher body manual.
If you need, i can post those pages.