I have just fixed a loud rear suspension noise on a '96 and a '97 SL2: not really a squeak but more like a loud squawk - sort of a rusty iron bed sound. Really bad, not subtle at all.
The cause in each car was a frozen ball joint in the rear sway bar link. Replacing the frozen link on each car fixed the noise.
The sway bar link is a dog bone shaped thingy about 5" long. Has a ball joint at each end, with a bolt sticking out of from the ball, a rubber boot around the bolt. The link goes from the sway bar end to a projection on the rear suspension knuckle.
I advise cleaning the rust from the exposed bolt threads and to use lots of Liquid Wrench, let soak overnight, and then try to remove the nuts. A real PITA with a floor jack and an ice cold driveway to kneel on.
A service garage fixed one car to the tune of $190 - $48 each for the links, $75 for labor.
I had him return the old links, one was still OK and I used it to replace the squeaky link on the other car.
It was very odd that they started squeaking within days of each other. Identical noise in each car, so I had figured that what had fixed one would fix the other.
Failure mode on the '97 was the loss of the sealing disk one joint. On the '97 it was the decay of both ball joint boots. In each case it was the driver side link that had gone. The good link on each car had no indication of deterioration - but I won't be surprised if in one week -- squeak!