AW-4 model changes.

I know that the output shaft spline count changed in '91 and I have heard that the AW-4 went to dual speed sensors (input/output) in '97. What is the benefit of the dual speed sensors?
I've noticed several instances where a newer AW-4 was installed with an older TCU. Could the opposite be done? Older AW-4 with '98 or newer TCU? The reason I ask is because the older trannies are easier to find and usually a lot cheaper.
Any other changes that were made to the tranny that are note worthy?

I'm not sure, but I know some transmissions use two speed sensors to sense clutch slippage. The newer computers can tell if a clutches are slipping and put it into a "limp-in" mode to protect the transmission from further damage.
Maybe that is what the two speed sensors are for?