What's vibrating? Entire truck,front end,corner,steering?
Could be a number of things. Out of balance tires/rims,bad belt in tire,out of balance driveshaft,ring and pinion backlash loose,alignment,steering parts (knuckle,wheel hub,tie rod ends,etc). If you put a 6" suspension lift on,it should have come with a drop down bracket kit for the transfer case to reduce any bad drive line angles. I have 6" Rough Country lift on my 2000 Tahoe and had no bad drive line angles at all. The body lift alone could be vibrating. Body lifts are so bad for vehicles. they just where cab bushings out prematurely. Most likely you have an alignment issue. Whenever you lift or lower anything,all specs go out the window.
What size tires you running? Find a good alignment shop and have them check your caster alignment. 2 degrees positive is factory spec normal. More details would be helpful.