Should be in the groove inside the side gear. Drawing below.
280ZXT's did not come with LSD. They have to have a 300ZXT LSD long nose R200 swapped in or an aftermarket LSD, like Quaife, installed in the R200 case. Not clear what you're working with. There are also 1988 300ZX viscous LSD's that use different length axles. And later 300ZX short nose viscous LSD's that also use different axles, but with the clip installed on the axle, not inside the side gear.
Here, "circular clip*". From the 1989 FSM.