Almost at an end of ideas....

[Ruthlessrico]

I have an 08 Sierra 1500 with a 6.0l engine. When I switch into 4wd I get a very significant vibration from somewhere. I can't even get over 50kmh without being terrified that it's gonna shake apart. When 4wd is not engaged there is a humming kind of sound that's consistent with the revolutions and speed. Within the last 20000km I have replaced the transmission, both cv joints, both hubs, full brake change, upper and lower ball joints/control arms and the u-joints in the front drive shaft as well as new tires. So I'm not thinking there could be many options left to cause the problem. Idea 1-some bearing in the front diff. A carrier bearing or pinion bearing? Not sure don't know much about them other than what they do. Idea 2-transfer case. No idea what part but something in there. Idea 3-a broken tranny or transfer case frame mount?? Think that might be a long shot. Or the last idea is that due to the 6" lift the front shaft doesn't fully insert into the case. And possibly has slowly worn away at the splines or has become unbalanced in some way. Either way any ideas or suggestions would be more than appreciated.