I have a 93 Lightning and the speedometer needle stay pretty still under 45, but once you get close to 45 it starts fluctuating. The faster you go the more varied the fluctuation. At 60 it moves back in forth at a range of nearly 20mph. This makes determining my speed almost impossible and my Cruise Control cant figure out what is going on and tries to speed up and slow down according the the speedo. Do these trucks have a speedo cable like the old school ones or does it have a Trans Speed Sensor?

Thanks

 

The Vehicle Speed Sensor is in the top front of the Diff.
It could be dirty or it could be going bad or there could be a problem with the tone ring on the carrier that it senses.
Try puling it and cleaning it, see if that solves the problem.

 

Read the TSBs in the bottom of this caption:

 

Thanks guys. I should have time on friday to check it out and we'll see what happens