idk...check the boots as stated above and look under your truck to make sure nothing is torn or grease coming out of!

 

I have yellow fluid drip from my tire every once and awhile, but it only seems to happen when the neighbors dog is outside.

 

Proably just windshield fluid, but man I'd get that noise isolated and get it checked out ASAP as it could lead to more seriouse issues, and even worse break down out in all that snow and you can't run the truck to keep warm till the tow truck gets there.

 

Ok.. on a serious note...

Not sure if anything is available where your at this time of year, but any chance you can WASH your truck?? if you can get the undercarrige clean, and more importantly free of snow/crud it would be easier to determine if you had a weird ice build up or if it was actually a mechanical part.

As for locations to check, I'd start at the Front output shaft on the Transfercase it can easily shovel snow right up to it.

Also since your "under load" in 4wd, I'd check the U-jounts on the front drive shaft. they can and will start clicking when they go bad.

Another thing you can try (if possible) is to take the truck out of 4wd and see if its still making the noise. Don't get yourself stuck or anything, but just hit up the local parking lot and give it a test on some easy snow packed ground.

If you've got a friend handy, bring him with, and have him stand outside the truck in the parking lot. see if maybe he can hear or see anything out of the ordinary.

 

What about muffler bearing grease?

 

I can never tell what color mine is; its always covered up with carbon and oil blow-by residue. I got a damn problem with my foot always being heavy.