Front end noise 2wd & 4wd

[blderman]

Trying to track down some noise I am getting from the front end of my truck. Starting at 20mph and continuing as fast as I can legally drive I get a "whirring" sound coming from what seems like the front drivers side. It doesn't matter if I am in drive, neutral, coasting, on the accelerator, turning, etc the noise stays consistent. If I put it into 4wd drive the noise becomes very loud and even louder on acceleration. Here is what I have replaced already:

Ball joints
U-Joints
Hub Assemblies (Timken)
Leaf Spring Bushings
Sway Bar Bushings
Serviced the auto hubs

I have even tried removing the hubs but that did not bring about any change. Today I removed the front drive shaft and found that the double cardan joint was pretty bad. Hoping that was what was making the noise I drove home and found that the noise remained. However, I briefly tried engaging 4wd and a horrible rattle came from the output shaft of the transfer case. I quickly turned the 4wd off. So ultimately I have two questions:

1) What is causing my mystery "whirring" noise
2) Is my transfer case bad or was it just not a good idea to run it without the front shaft?

Thanks!