GMC Repair: front end vibration, front end vibration, vibration analyzer

Question
I own a 2000 GMC Sierra and have been noticing that at about 40 mph I pick up a vibration in the front end.  The faster I go the worse it gets.  I have already replaced the u-joints on the driveshaft, balanced and aligned all tires, and bought new tires trying to eliminated the vibration.  I noticed that when in four wheel drive the vibration is much worse and almost feels like the front end is going to come apart.  First of all,  have you heard of anything like this before?  and second,  what do I do next?

Answer
Hi
I would have the vibration diagnosed by using an EVA (Electronic Vibration Analyzer).
It will give you the frequency of the vibration, and from that and using Vibrate software, the vibration can be pinpointed to the general location of the cause (Tires/wheels, prop shaft, engine, etc...)
At this point, that would be my advice.
It is possible that the front axle OR the transfer case could be creating the vibration. Bad mount? Broken or loose mounting flange(s)?
I Hope this helps
Pawl