Find a bolt with a 13MM head and jamb 2 nuts on it. Use a box wrench on the jambed nuts and you should be able to open the differential.
Consider some heat on the plug bolt to help loosen it.
Consider a grade 8 bolt for this, or a metric equivilent.
A grade 8 bolt has a rating of 150,000psi. A metric grade 10K bolt is 142,000psi and 12K bolt is rated at 170,674psi.You may have a hard time finding a 12K bolt. But you should be able to find a 10K bolt.
Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car