GM has had lots of problems with the gauge clusters on these trucks. GM has a special policy that covers some or all of the repair costs if you have less than 80,000 miles on your truck. If you have more than 80,001 miles you are on your own. The instrument cluster needs to be replaced. Removal is easy. Apply the park brake and move the shift lever all the way down to the 1 position. Then remove the bezel around the dash panel by pulling straight back towards you. There you will find four 7mm screws holding the cluster in place. There are several sellers on ebay that have remanufactured clusters starting at about $150. These will even come programmed with the proper mileage reading for your vehicle.
Here is the full procedure for removal of the bezel. There are no tabs to push in or release . All of the retainers release themselves as you pull back on the bezel.
As far as where to get the replacement cluster, I can say that Autocraft is a good company and have had no problems with their products. I believe their replacement clusters run about $350 and will need to be programmed with the mileage at the dealership.
There are also several sellers on ebay. I cannot vouch for the quality of their products, but the prices are far better and the cluster comes pre-programmed with the mileage.