My speedo went haywire yesterday and started bouncing all around.Best part was it was on a 8 hr trip back from Oklahoma, so trying to figure out speed was fun. So I know I have to replace the speed sensor, but the tack was moving a little too. The engine was running fine and no RPM spikes, thought it was a little wierd but figured it had to do with something in the gauge cluster.

 

the speed sensor I believe on your truck is on the rear pumpkin and reads a ring gear that is meshed between the ring and the carrier. you will see it at the top with some wires comeing out of it. if thats not it it could be your gage cluster.

 

Thanks