Weird suspension sag. Help please

I have a weird thing that I can’t figure out. My rear left suspension sags.. but only when I park it for more than an hour. As soon as I drive and hit a few bumps, it levels out and acts completely normal, but when I go into a store and come back out, it’s sagging again. I tested this by measuring before I park, and then after, where it drops multiple inches, then I purposely roll over a curb and it’s sits normal. it doesn’t sag when I’m driving or off-roading. Literally just when it sits still for more than an hour. I’ve never heard of this sort of thing and I want to see if any of you can help troubleshoot this before I spend too much money taking apart my suspension and buying stuff I don’t need to. I have bilstein 5100s in the rear and stock leafs.

 

The seals are probably leaking slow and the bumps pick them back up when you start driving, but don’t leak out fast enough to start sagging before you hit the next bump. I can’t imagine the leafs would cause this or what else.

 

That was my first thought joe. I need to look into it more. and I don’t have real measurements yet but I measured with my hand and it’s pretty obvious.

 

Shocks aren’t really what hold the rear of the truck up. They absorb “shock” for the most part.
The majority of the weight is on the leaf spring.
I suppose you could measure the shock length of each side after it sags?
Then you’d know if it’s a shock/spring issue.

If I were to guess, and this is total guess. (I have no idea)
Maybe you have a shock that doesn’t fully “decompress” after the truck rebounds over a bump. Or maybe you have a weak leaf pack?
Have you checked for a broken leaf?