O/D OFF on a 1993 LS 400

I have a 1993 LS 400. The O/D OFF indicator is flashing on and off. A transmission shop (Amoco) did a computer read and says the Speed Sensor needs to be replaced.

Anybody has similar experience ?

How difficult is it to replace the speed sensor ? Where to get it besides the dealer ?

Thanks for any suggestions.

Joe Dublin, OH