The rear rotors can sometimes be stubborn. Even on my unrusted California cars I've had to use some persuasion to get them off. Penetrating oil, a torch, and like PEH said, a couple of good whacks with a big hammer should get them off. Be a bit careful with the penetrating oil. You don't want it on the calipers or the bearings (don't worry about the rotor, you're replacing that anyway.)

By the way, on the 123 cars there is a single phillips head screw that holds the rear rotor in place. I assume if your car has that you've removed it.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold