Ok and what's the problem? It is a Jeep, they clank,rattle, squeak, squawk, squeal, it's a jeep. LOL!!!
Anyhow, most likely its a mount or the item mounted to it that is doing it.
When the weight on it is unsprung it's able to move enough to squeak.
You toss your fat friend in the back, and his weight slightly compresses stuff, like the springs/shocks, and it changes the angles on the control arms and rest of the suspension. They can't move around as freely so they can't squeak.
Get under it, shoot every bushing you can find with something like PB Blaster.
Then check every single mount bolt/nut to be sure they are tight. Look at the mounts themselves too, you may have one that's cracked and when it doesn't have weight on it, it moves slightly.
It's either that or haul your bud around in the back.