My '87 wagon does the exact same thing. Kind of a rubbery
creaking sound. Goes away when it's warm out; anything
above perhaps 60F. Reappears whenever the temps drop below
Can't say I've put much effort into diagnosing it. I
kind of assume it's dried out old bushings in the
rear suspension. I understand the sway bar end links
in back are notorious for wearing out and making noise,
but I think it's a different character of noise.
I've noticed the noise disappears when driving in the
rain, even if the temperature is quite cold. This
reinforces my belief that it is dry bushings. Lubricate
them with a little rain water and they quiet right down.
Don't know if there are specific bushings which must be
replaced to quiet down the car, or if the whole rear
suspension should be done. Maybe one of the techs with
some 124 experience can comment?