2010 Silverado 2500HD 4x4 vibration and hum

[Huffy454]

I have a 2010 Silverado 2500HD LT crew cab 4x4. It started getting a shake after I put new wheels and tires on it, found out they were bent. Had a lift installed and new wheels and tires that aren't bent, still has a shake. Replaced U-Joints on driveshaft, 90% of shake went away. Now when I'm in third gear over 3500 RPM there is a vibration in the floor board and a hum, but it goes away when it shifts, but comes back once I get over 3500 RPM in the gears after third. Once I hit 70 MPH it starts to vibrate and hum and gets worse the faster I go. Any ideas what may be causing it?