I've always heard that in order to use a tach/shift light on a J you gotta buy the adapter to read correctly.
When I bought the car it had a tach(long since removed).
Well I followed the signal wire into the engine bay looking for the adapter box...
Went into the passenger fender well by the ECM.. and is somehow hooked up to the ECM.

Anyone ever done this or know anything about running a tach/shift light this way?