OK guys I may be the bearer of bad news you may not want to hear.
On these trucks there is a a sensation called "frame beaming" that can be felt between 50-70 mph...this is a normal condition and there is a GM Technical bulletin regarding this condition.
In a nutshell your truck was designed to be used as a truck (you know carry, tow and haul stuff). When it is driven unloaded this sensation is prevelent.
The bulletin has a diagnostic procedure to drive the vehicle unloaded with an Electronic Vibration Analizer and note the frequency of the vibration. If the frequency falls within a certain range, the next step is to add approximately 500lbs of weight to the vehicle and reroadtest it again. If the vibration is not present with weight in the vehicle then this is a normal operational characteristic of the vehicle and no repair attempts are to be taken.
The sensation you are feeling is most likey more noticable when you are driving on concrete surfaced roads..the reason for this has to do with suspension geometry, driveline angles, wheelbase and the harmonics of the vehicle tires hitting the expansion joints of the road surface.
With no weight in the vehicle, there is nothing to dampen these vibrations and they will transmit into the passenger compartment.
Now before you start argueing with me about this, I own one of these vehicles and have felt this sensation with my vehicle..most noticable at highway speeds on concrete surfaced roads with the truck unloaded.
Since I have loaded my inbed toolbox with my carpentry tools (about 300-400 lbs of weight) I have not felt this sensation.
My advise to you is this.
If you are feeling this, it may be a normal characteristic of your vehicle so be prepared to hear this from the service department.
But, make sure that the service department does a bulletin search and that they drive your vehicle with the Electronic Vibration Analizer to insure that there are not any addressable vibrations on your vehicle.
The EVA is a essential tool that all GM dealers should have in the service department and there are service and diagnostic procedures in service information in regards to vibration diagnostics.