Check your RPO label underneath your trunk lid. The rear end codes start with 'G'. Make sure to check the vin on the label matches the vin on your car.
Rear end codes for G-bodies:
GH2-2.29 ratio
GM8-2.56 ratio
GT4-3.73 ratio
GU1-2.41 ratio
GU2-2.73 ratio
GU4-3.08 ratio
GU5-3.23 ratio
G72-2.14 ratio
G80-limited slip differential
Rear end codes for G-bodies:
GH2-2.29 ratio
GM8-2.56 ratio
GT4-3.73 ratio
GU1-2.41 ratio
GU2-2.73 ratio
GU4-3.08 ratio
GU5-3.23 ratio
G72-2.14 ratio
G80-limited slip differential