At any road speed, the speedometer may read correct or it may read 30 to 50 MPH over. (Ex: 45MPH actual, reads 70 to 90MPH)
Or the needle may be swinging all over the range. This is a high mileage truck and has had 3 or 4 owners. I have owned it for two years and use it just for occasional local light hauling.
Any ideas? Thanks.

GM has a service magazine on this problem, not covered unless under warranty. You have to pull the gauge cluster out and have the dealer send it to a repair facility and they replace some diodes. Mine does the same thing I just live with it. I was quoted $300 to fix mine 4 years ago.