GMC Vibration

[AllTerrain15]

August of 2018 I had my transfer case and front differential rebuilt on my 2015 Sierra All Terrain due to violent vibrations going down the highway. Since then ive noticed moderate vibration that is progressively getting worse. I notice it more at low speeds when it is shifting but it is present at all speeds. It does not start vibrating until transmission is warmed up. Dealer recommended new tires and front end alignment, so i got that done but the problem remained. It is now back at the dealer for the second time since transfer case was rebuilt. Shop foreman still thinks its tires but i just cant see it. Im thinking maybe a u-joint issue or rear end issue. Any input would be appreciated.