[silicon212]

Not saying this is definitely what you have, but every B-body CHEVROLET I've seen from the 80s has the G87 in it (8.5"). I've yet to see one with G89 (7.5") in it. Your '84 should have the SPID label in the trunk - look for any "G" RPO codes -

GH2 76 87 AXLE,RR 2.29 RATIO
GH3 86 88 AXLE,RR 2.77 RATIO
GH4 76 81 AXLE,RR 2.28 RATIO
GL1 76 81 DIFFERENTIAL CARRIER,RA AXLE,ALUM
GM1 87 87 AXLE,RR 2.59 RATIO
GM3 85 88 AXLE,RR 3.45 RATIO
GM7 82 83 AXLE.RR 2.39 RATIO
GM8 76 88 AXLE,RR 2.56 RATIO
GN3 76 81 DRIVER CONTROL LEVELING
GS4 85 86 AXLE,RR 3.70 RATIO
GT4 76 88 AXLE,RR 3.73 RATIO
GT5 76 88 AXLE,RR 4.10 RATIO
GT8 76 88 AXLE,H.D. RR 4.10 RATIO
GU2 76 88 AXLE,RR 2.73 RATIO
GU3 82 85 AXLE.AR 2.93 RATIO
GU4 76 88 AXLE,RR 3.08 RATIO
GU5 76 88 AXLE,RR 3.23 RATIO
GU6 76 88 AXLE,RR 3.42 RATIO
GV4 82 87 AXLE,RR 3.36 RATIO
GV8 82 85 AXLE,RR 2.72 RATIO
GW4 84 85 AXLE,RR 3.31 RATIO
GW6 85 88 AXLE,RR 3.27 RATIO
GW9 76 88 AXLE,RR 2.93 RATIO
GX3 82 88 AXLE,FRT 3.33 RATIO
GX4 76 83 AXLE,RR 3.75 RATIO
GX5 76 83 AXLE,RR 4.09 RATIO
GY5 76 87 AXLE,FRT 3.65 RATIO
G44 85 87 AXLE,RR 3.07 RATIO
G50 76 88 SPRING,H.D. RR(VAR 1)
G51 76 87 SPRING,SPL H.D. RR(VAR 2)
G52 82 87 SPRING,EXTRA CAPACITY RR
G60 76 88 SPRING,AUX
G66 76 88 ABSORBER,AIR BOOSTER RR SHK
G67 76 77 ABSORBER,AUTO LVL CONT SHK SYS
G67 78 88 ABSORBER,ELEC LVL CONT SHK SYS
G72 80 80 AXLE,RR 2.14 RATIO
G72 81 81 AXLE,RR 2.14 RATIO (EXPORT)
G72 82 87 AXLE,RR 2.14 RATIO
G75 82 87 AXLE,RR 3.62 RATIO
G80 76 88 AXLE,RR LOCKING TYPE
G83 82 83 GEAR,RING 8.75 IN
G87 76 87 GEAR,RING 8.50 IN
G88 76 83 GEAR,RING 8,875 IN
G89 76 87 GEAR,RING 7.50 IN
G92 76 88 AXLE,PERFORMANCE RATIO
G95 85 88 AXLE,ECONOMY RATIO