You should have a ton of salvage yards in the Los Angeles area.
auto salvage in los angeles area - Google Maps
I googled this, hope there is a differential in one of these places. plus with all the towing co`s dotted across the map, one of them might have one in their impound yard.
You need to keep checking back weekly, as the inventory changes constantly. there are a lot of the 3:07 from 82 -84. If you use one of these, then change the spedo from that donor car. the SD will also fit, but the spedo will not fit into a 123 dash, would need one from a W123.
Iam with Dropnosky, no way a 4-spd manual is going to damage the Diff. I have a 4-spd behind my 85 300D.
Charlie