Need help on my Silverado.. Grinding.. Whirring sound up front in 4WD

I'd look towards the gearbox, have you checked the fluids in it? Might be a good time to change it. Let the first quart go into a clear plastic qt jar or something, then comb through it for metal bits, maybe just let it settle then pour it off the top till you get down to the bottom so you can know if anythings breaking loose. Also poke your finger up on there if possible to check for anything loose that didn't come out with the oil.
After that I'd look on downstream till you find it