Awesome, thanks everyone.
I had a chance to mess around with the sliding door a bit.
Took off the cover to the top outside slider and found some rust under whatever the previous owner goobered into the drip rails. Nothing unexpected though.
Lubed up the sliders and hinge mechanism. Happy to see GM put a couple of zircs on the hinge.
Now the doors slide smoothly and much more quietly. Noticed something that looked like a latch in the upper hinge, parts 22 and 4 in this diagram.
Does anyone know what the purpose of this is?
I'm still not happy with how the door opens and shuts. Found a thread about using pex tubing to replace door striker bushings on GM trucks of the same era:
https://www.gmsquarebody.com/threads/fix-slamming-doors-striker-door-latch-repair.624/Will give this a try soon