One side lower than the other
 

I recently found that my rear driver side is 1 1/4 inches lower than the rear passenger side.

Some context:
- This is not a daily car, so, only goes for some light/medium drifting sessions or rides ~twice a month.
- Rear shocks were replaced ~ 1 year ago (I didn't pay attention on the height at that moment, and recently I was out of drifting for some time, so no wheel changes were done for a long time to notice the issue)
- Welded diff

As I have some tools, I first thought one of the springs was worn out, or maybe the bushings, or even the lower spring bushing was not installed the correct way when replaced the shocks. Anyways, I removed both springs and bushings to find everything looked fine. I then switched the springs and bushings so if some was worn out, the change would be visible after putting the car back on ground. Surprise to me, the driver side is keeping the same height difference (1 and 1/4 inch lower).

Next thing I was planning to switch was the rear shocks (but since preload is mostly responsibility of the springs, I'm not sure if that worth the effort..)
Also, I was thinking to remove the rear shocks and put the car on the ground to see if there is a change. Would that be something possible? I'm 60% sure nothing will happen just to put the car on the ground without rear shocks, but want to confirm with you guys.

Any idea about what else I could check by myself before going to a shop?

Perfectly flat concrete with a full tank of gas ? Cause yes it matters

My garage is pretty flat (I don't believe in perfection :icon44:), and also in different places where it "should" lean on the passenger side.
Full, 3/4, 1/2 and 1/4 fuel tank..

I was also reading the sway bar could be the culprit in case it was (re)installed with the car on stands...
I will also try to take it out and put it back while all wheels are on the ground.