Ok, next update for anyone still following this thread. The double hex nut arrived today, so after dinner I spent about an hour on the car. The new pinion seal is in with sealant, the flange is in (put a thin layer of grease on the seal running surface) and tapped it in flush with a hammer. The new double hex flange nut is in as well, torqued to 185 Nm, and I checked the turning torque and it is set to the same value it was (if not a touch more) when I checked it before taking it out. I'm going to let the sealant dry overnight before I fill it with gear oil and see if it leaks statically like it did before. If it stays dry, I will peen the flange nut and put it all together again.

Wish me luck folks, the factory instructions were followed and measurements were matched as much as I possibly could. If it still leaks my next course is to rip the differential out and put in another one.

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2012 Mercedes ML350 Bluetec 91K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 231K (mine)
1998 Volvo S70 T5 Turbo 196K (kids)
1994 Ford F150 4WD 249K (firewood hauler)
1983 Mercedes 300D 376K (diesel commuter)