G80 won't be on the axle itself.
You'd only find that listed on a buildsheet or RPO label.
It's easiest to just spin the yoke and see if the axles spin in either the same direction (posi) or opposite directions (open).
There are different codes on the front of the passenger side axle tube, depicting what gear ratio it has and whether it is a posi rear. An open 3.73 rear and a posi 3.73 rear will not have the same code.
Given it's age and unknown condition, you should be pulling the cover off anyways to not only change the fluid, but you can confirm gear ratio and the presence of the posi unit.