Vibration at 80-95kmph (06 Silverado 2500)

[Peakey91]

I own a 2006 Silverado 2500 HD, (6L Gas) and when I am travelling at around 80-95km I get a terrible vibration in my 4wd gear selector. I only get the vibration when im coasting not accelerating. Usually when I'm going down hill it's the worst. As soon as I put my foot on the gas it almost makes a snapping noise and the vibration goes away. It's been getting a lot worse. I needed a bunch of work done so I did my tie rod ends, my pitman arm, LH hub/wheel bearing, front drive shaft u joints, (about to do rear shaft u joints) I also noticed trouble shifting into 4wd when my front end was lifted off the ground it made a loud snapping noise. Can't remember specific details about that though. I wonder if my transfer case is on the way out

[Peakey91]

I realize now I posted this in the wrong spot haha, obviously new to this