If you have the stock GMC hitch, it will have a round bar from one side of the frame to the other, (drivers to passenger side) and is made of thinwall tubing, then heat treated to give it some strength. With time, these crack, and the Hensley or Propride hitch will put additional stress on the stock hitch.
So if you have a stock hitch, replace it soon, or it will come off suddenly, and require trailer and hitch replacement as well.
At least check the hitch closely for cracks, or rusty lines, and perhaps have it inspected by someone. It might be a poor hitch, that allows the weight distribution bars to wiggle, that is the root of your problem.
One Rv.Net member measured his hitch with and without the weight distribution bars on. He found he back end of hitch is moving about 0.040" when the bars are tightened. With the replacement hitch, it did not move a significant amount.
Fred.