I have some recent experience regarding speedometers/speed sensors.
Unless your situation is unusual, I think your problem is in the
speedometer head - not the sensor.  

FWIW, on my '94 pathfinder, the sensor is located on the bottom of the
transmission, on the passenger side.  There are a few sensors there.
The speed sensor one was the closest to the front of the vehicle.  In
my case, a new sensor was $60 but I found one at a junk yard for $30.
Replacing it didn't solve the problem.  I traced the problem to a bad
IC in the speedometer head.  Unfortunately, this IC is not available.

If this matches your situation, you might want to check junkyards or
Ebay for a new speedometer head.  However, be careful as the
speedometer head is a weakness of Nissans of this vintage.  In my
case, I was able to implement a replacement circuit for the defective
IC.

Marko