When I purchased my 1996, the speedometer and the odometer didn't work at all. The check engine light was on and it read p0500 for no input from the speed sensor. I replaced the vehicle speed sensor with one from NAPA. It came with a new connector and some length of wire. I just ignored the extra pigtail and put the speed sensor into the transfer case.

It worked! The speedometer and odometer sprang to life with the new VSS. I've driven about 80 miles so far when I get onto the freeway and up to about 65 mph. I look down, and the speedometer is going crazy. It was all the way up to 85, then it was completely south and off the readings. After a few miles of it going up and down at a frightening rate, it stopped working. Occasionally, it would spring to life and shoot way up and down a few times for a couple seconds and fall dead.

Umm... what the hell is happening?