It is either pull the tranny or pull the engine. Consider yourself lucky, it could be a leak from the left head gasket.

Seriously, you need to have that looked at as well, the oil can run down onto the lower bellhousing and look quite similar to a leaky rear main.

My 420 has a slightly seeping left head gasket, someday I see myself pulling the whole engine and doing a complete reseal job.

Chas