Can the center (drag) link go bad?

Yes, of course.  ANYTHING CAN go bad.

my understanding those 'dial' regulators the numbers don't necessarily relate to psi ....

and you replaced the crush collar, did you check pinion bearing preload as you tightened the pinion nut?

was it growling before the crush collar or after?

The pinion bearing needs some preload, and the crush washer is supposed to be sized and held tight to the amount designed for. I don't know the affect of loosening the nut after crushing it slightly too much, that's an unknown. I haven't ever set a pinion bearing, so I don't know enough to judge.

I would watch the gear oil level closely, be sure it's very high quality synthetic, and considering the noise, maybe check the gear lash inside etc.

That's the clearance of the ring to the pinion gear, about .012" typically. The 8.8 is supposed to be in a  range of .009" to .014 if I recall, I aimed for .011 or .012" and hit .012" with the last two I have done(Crown Vic and Explorer).

The pinion gear is supported by three bearings, that is one big key to a 9" rear, it's very very strong. But checking it I'm not sure of how to, looking at gear wear is a poor mans clue check. If anything makes you worry, you might have to take it back apart to reset check and reset the pinion bearing preload(and crush washer or pre-crushed sleeve).

That's true, the costs are high and the 9" is hard to R&R. I did it a few times when I was far younger, twice in junkyards.

I think it might be good to consider a swap of the chunk. Hunt for one that you need more, try to plot the trans swap if that could happen. If you moved to an OD trans, then you could use a different gear no matter what. I've got a 3.70 and a 4.11, just LS's in my trucks. Way back when a used stock 9" was $25, and a little more if it had an LS. Now they all are valuable, you have to look harder these days.

So I'd been having issues with the secondary dripping from the venturis when not in use, no matter how much I lowered the float. It made it hard to start after sitting a few minutes, and it was made worse by attempting a full-throttle takeoff from a stop, which next time I slowed down flooded the engine so much it tried to idle at 200 RPM before dying. So figuring there was crap in the needle and seat, I found a rebuild kit at O'Reilly that included new needle/seat assemblies, and took the carb apart. The secondary needle/seat was a bit sticky. I got it moving a bit better with carburetor cleaner, but I went ahead and replaced the needle and seat, and also sprayed cleaner through all the passages I could find. One of the air bleed holes was clogged also, so I took a needle meant to clear the nozzle on a 3D printer and poked it through, which then allowed carb cleaner through to clean it the rest of the way. The venturi that had been dripping more was the same side as the clogged air bleed hole, so that might be related...? I replaced the primary needle/seat since I had it, and I cleaned all those passages as well. I then reset the float levels and installed the carb with a new fuel filter.