[fastpat]

It looks like you have a diff out of an E30 4 cylinder car, they're a small diff. In order to get a diff that will fit your E21 6-series, you need one from an E30 6-cylinder car, an E23 7-series, an E28 5-series, or another E21.

Then it's a simple matter of swaping the diff covers, swapping the stub axles (and maybe renewing the side seals, cheap), bolting it in (use a tranny jack), and hooking up the speedometer sensor.

Pulling the broken diff apart to replace the bad part requires a rebuild, at around $800.00. Not the thing to do.