When I removed the passenger side half shaft with the car in the air and no support under the tires, the trailing arm with the wheel assembly dropped down several inches. My thought was maybe? The u-joint needle bearings inside of the cups were destroyed thus creating play at the joint itself. If this is the case? Maybe there is enough slop to cause the trailing arm assembly to drop slightly as seen in one of your photos.

Also check to see if the hub bearings on the passenger side are ok? To get a good look at these bearings without taking everything apart, about the only mechanical you'll have to do is you will have to remove the passenger side half shaft to unload the suspension. Now, with the half shaft out, you can get a better look to see if there is any slop within the u-joint itself. Once the half shaft is removed you can spin the wheel to listen if there is any unusual noise. Also grab the tire at the bottom and top to wiggle up and down, in and out to see if there is any major play. The tolerance in the hub bearings when properly setup are very minimal.

You may even want to try this first? With the lift you're working off of, you can even load and unload the assembled passenger side suspension to see if anything obvious jumps out at you while watching the trailing arm and suspension as you load and unload it? Is anything moving that shows a lot of slop? Or moving that shouldn't be? At first I was thinking something may be bent? But your photos show the passenger side hangs lower than the driver side with both sides up in the air. You also stated that the passenger side sits lower when all four tires are on the ground with the suspension equally loaded. I may be wrong? But this indicates a common denominator that affects the passenger side suspension while being loaded up, and unloaded. I believe you have some slop on the passenger side with some suspension part? Worse comes to worse? Can you spin the spring 180 degrees to see if there is any difference to see if the problem moves to the driver side?

Just some ideas of how I would approach this if your car was at my shop. It may be a lot of turning wrenches and aggravation, but the parts list and expense is minimal. You may want to buy a set of four French locks for the back of the hub flange bolts unless yours are still in good shape.

I'm sure with the suggestions and help from others on this site, you will find this issue and solve it! Once you find it, hopefully you will let us know so we can all learn.
Good Luck!

 

 

Have diff level

Clearly right side not hanging as low

Look at gap to shock for reference

Right side sitting on frame

Left side sitting on frame

 

The bushings were shot - your diff and your butt will thank you.
I would:
Check the shocks while it's apart.

Are both trailing arms hanging down the same amount?

Remember that the 1/2 shafts cannot turn when they are way down due to the angle in the universals. If you want to test then bring them up to almost level.

See if you can put a level across the diff cross member at the diff mounting point to see if the cross member is bent. Easier to do it now then later.

Great work. The end result will make you happy.

 

 

 

Check out this beauty. LOL

 

 

In photo number 4 from the top it looks like the arch of the spring is higher on the right side that the left side. Could the spring itself be tweaked or weaker on one side than the other? A bad spring maybe?

 

 

In the photo of the rear with the level the left lower mount looks off - it may be the photo angle.

I would take the spring out and after the rear is remounted check the mount for level and / or bends.

The spring should hang almost level.

I'm often too exacting so I would be cleaning and measuring each part as I went. IT's easier now then to take it apart later.

Unless something is binding the spring should set the ride height. I would check each part going through it's movement to see if anything binds.

 

When you say this, do you mean that the attaching bracket on the "nose" of the differential is also disconnected from the frame?
Since this is becoming quite the puzzle, just out of curiosity, have you moved the jack that you have under the differential? The ring gear, which is one of the heavier components in the diff case, is offset, which may be contributing to the last bit of "droop", now that most everything else has been disconnected.

Just thinking out loud....

 

 

 

The strut bracket looks like it's in the correct position, flat side to the rear of the car. Check out Globalwest.com and watch Dougs Video on strut rods. I've installed these on my rebuild/yet to be tested, but I'm confident they'll do the job.

 

Had the same issue but driver side was low, new front springs and the problem went away.

 

Just a point not that it applies here but sway bar adjustments can effect ride height side to side.

 

I got mine from RICKS CORVETTE, They make alignment easier. They are also more durable if you intend to really push the car. If yoy are going for an alignment in the rear I would get the SS shims for the rear trailing arms - 2 packs.

This is why I replaced mine.