They don't make that convenient, do they? I don't have a suburban and couldn't picture how it was all set up in that area.
What does it look like from inside the truck looking back at the area where the bolt goes through that pillar?
The GCVW on the truck is going to be your issue.
https://www.gmheritagecenter.com/docs/gm-heritage-archive/vehicle-information-kits/Chevrolet-Trucks/1987-Chevrolet-Truck.pdf
I had the same issue with the spring off. I was working on it and forgot to hook the spring back up and as soon as it started it was probably 3000 rpm. Everything was fine after hooking it back up correctly.
The driver's side always sags. I think they starting sagging on day 2 so I think you are onto something with the previous owner trying to correct that.
Some of the ford E series class C motorhome chassis came with a tag axle. It used the regular dually wheel like the rest of the truck. Same bolt pattern as the GM stuff too. I used my ex father in laws Holiday Rambler class C with the tag a lot and it really helped stabilize it with that big...