More info on gears:
Rear
H190(AL) Type I
520 from 12/71
620 up to 9/73
H190(ML) Type II
620 from 10/73
720 2wd (except Z24i & HD)
720 4x4 up to 6/83
D21 2wd 4cyl A/T up to 12/85 & M/T up to 1/86
H190(ML) (12mm ring bolts)
D21 2wd 4cyl A/T from 12/85 & M/T from 1/86
C200 (10mm ring bolts)
720 2wd (Z24i & HD)
720 4x4 from 7/83
D21 4x4 4cyl up to 4/86
C200 (12mm ring bolts)
D21 4x4 4cyl from 4/86 up to 1/92
WD21 4x4 4cyl from 4/86 up to 1/92
H233B (10mm ring bolts)
D21 V6 up to 6/86
H233B (12mm ring bolts)
D21 V6 from 6/86 up to 8/89
D21 V6 from 8/89 up to 1/92 - different than above
D21 V6 and 4x4 4cyl from 1/92 - different than above
WD21 V6 from 6/86 up to 8/89
WD21 V6 from 8/89 up to 1/92 - different than above
WD21 V6 and 4x4 4cyl from 1/92 - different than above
Front End Diffs:
R180
720 4x4
R180A (10mm ring bolts)
D21 4x4 4cyl up to 6/86
R180A (11mm ring bolts)
D21 4x4 4cyl from 6/86
WD21 4x4 4cyl from 6/86
R200A (10mm ring bolts)
D21 4x4 V6 up to 4/86
R200A (12mm ring bolts)
D21 4x4 V6 from 4/86
The pinion bearings are different on the pre 90 stuff. 90-2004 is basically all interchangable.