My understanding is that in an automatic car, the tach pickup gets its signal from the ring gear. Now that there's a .411 in place of the auto trans, how do I get the tach going again?

This is correct the location of the pickup is rear engine oil pan drivers side. That feeds a signal to the ECU which in turn drives the tach.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10