Well after searching and going thru 100s of pages I have a guesstimate of what it may be, but....

Getting a wah wah wah sound, wah gets slower as wheel rotation slows. Running MT's so I really don't notice it till I let off the throttle and I am coasting. Noticing it getting progressively worse. I am a couple thousand past my tire rotation that I normally adhere to, but highly doubt that is the problem. U joints were replaced around 20k ago, but I know that sound. No shuttering like when the drive shaft joints go out. Never have heard this sound before after putting 120k on this truck since new.

Any suggestions? Driving back to ID next week and a little concerned, but thought I could take it to my diesel tech there since I know of none in Bozeman.

 

Any wierd wear issues with the tires? Jack up the front wheels one at a time and check for any slop in the hub bearings, and any strange noises as you turn them.

 

Might be the carrier bearing. Mine made a very subtle whirring noise for about 15K miles before it finally went out. The sound only changed with vehicle speed, not engine speed. Couldn't identify what the noise was, because it would only start up after driving a couple miles. Couldn't get it to make the noise while on jacks. It sounded like the speedo cable was making noise. Kind of a warbling sound. The carrier bearing never felt hot, even after a long drive, which is what threw me. Then one day it let loose. I thought all H#!! had broke loose. Felt like my front end had fallen out.

Now I have a one-piece driveshaft.