2002 ML55 AMG, 5.5 liters
Yes, I replaced rear springs with a set I bought on ebay. The shocks where bad so I replaced them with my bilstein shocks. The rear now rides a little rough compared to the OEM shocks but I can live with it and the bilstein really shows when I'm off road.An 2003 ml 55 would in had 285/50-18
Circumference is 29.2. & 11.2 wide
Your 275/45 -20 =
29.7 and 10.8 wide.
So It seems that tire size should be fine.
You check for a broken rear spring? If I remember correctly you recently were looking to change them?