Nope M8, the 12 sided nut the holds the pinion in the diff itself.

Looks like its might be either 36mm or 38mm but I can't get it off untill I make a tool to suit but I need a 12 sided socket 1st to fabricate a tool.

The 'other' nut you are on about is just a 24mm locknut that holds the driveflange on, that ain't a problem