A Limited Slip Differential (22% traction, 27% coasting) is now available for the MY2004 996 C2 as an option. Option Code is M220. Price tag in Germany is 1102,00 Euro (incl. 16% VAT).

Apparently the LSD is not available for cars with the 30 mm sport suspension (X74) but I'm not sure if this is a mistake in the data I got.

The LSD option is not available for the 996 Targa or 996 C2 Cabriolet. It is also not available for Tiptronic equipped cars.
PSM, M030 and 18'' wheels are mandatory for the LSD option! The LSD is not available without PSM!