More technical people will chime in but I think you can drive it while it is leaking - just keep an eye on the oil level. The rear main seal keeps the engine oil in the lower part of the engine and I don't think it should affect the transmission. Working on the head has nothing to do with the rear main seal (I think). Mine is currenlty leaking a little but it has not increased in almost 30,000 miles.

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dtf
1994 E320 Wagon (Died @ 308,669 miles)
1995 E300 Diesel (228,000)
1999 E300 Turbodiesel ( died @ 255,000)
2006 Toyota Tundra SR5 AC 4X4 (115,000 miles) rusted frame - sold to chop shop
2011 Audi A4 Avant (165,000 miles) Seized engine - donated to Salvation Army
BMW 330 xi 6 speed manual (175,034 miles)
2014 E350 4Matic Wagon 128,000 miles
2018 Dodge Ram 21,000 miles