I think it's on the right side of the transmission. It can be reached on my car without to much trouble.
Just unplug the molded plug and remove the single mounting bolt and it should come right out.
It might be referred to as a speed sensor.
I don't really know what the part number is but a GM dealer should know.
A transmission shop might also.
I'd sure replace it before you go on your trip.
I had to limp home in first gear. It took forever but at least I wasn't stranded.

DT