D21 had two rear diff types. C200 and H233b.
The c200 is the common as hell one and is similar to the r200 in parts and construction. the r200 is found in Silvia's and Skylines and is a solid diff.
the H233b is a beffy unit with parts similar to patrol. These are big units and preferd upgrade.
now the issue with these two is, driveshaft flange is different so you cant direct swap unless you get the tailshaft too. It is possible to swap the diff pinion flange over but i found there were two types of h233b one with a compatible flange and one without - so its hard to tell and work out...you best of swaping same for same, unless you get a good deal on the diff+tailshaft.
Now with ratios, regardless of c200 or h233 they all came out in a handfull of ratios. So to maintain standard drive you really need to ensure for yourself it has the same ratio.
In your engine bay is your compliance plate and under transaxle it will say a two letter code followed by two numbers. This is your diff type and ratio type.
CA46 - CA = C200, 46 MEANS 4.6 ratio diff.
HG43 - H233b with 4.3 diff ratio.
If you cant compare build plates you will need to spin the input shaft and count the rotations...to give you the ratio.
Attached is a build plate showing the transaxle information.
side note : d22 diffs are longer by 4cm each side and wont bolt up.
