Changing differential fluid

My 2006 silverado 4x4 has been making a low humming/ grinding sound while driving. It has sure to been at least 35000 to 40000 miles since it has started. I have changed fluids at 50000 miles and recently changed the front diff fluid due to a milky appearance most likely due to an improper seal when the dealer changed the fluids. The noise is a low deep sound that is always in intervals and always the same. The only time it changes is sometimes it isnt there at all. However, at highway speeds 55- 75 it gets longer in the time the sound is made and the intervals are a but longer. At lower speeds the noise is in shorter intervals and is a shorter 'hum". The noise is made with or without the engine running and also with the transmission in N. Both front hub assemblies have been changed and I put on aftermarket because the stock hubs busted. I know its not those bearings. What could this be? Please help.

No, in fact the sound began before that. I changed the transfer case fluid today and still the same. All fluids have been changed. I suspect it must be the front diff, rear diff, or thansmission areas. But I can feel it in the steering ever so slightly so I must be in the front right?

My truck has about 170,000 miles and the transfer case and differential fluid has never been changed. If I change them now can it cause any harm? Also, what kind of fluid goes in the two?

Just want to make sure I am getting the right stuff and amounts

for the front differential I need use 80w-90(how many quarts) and for the rear differential I need use 75w-90 synthetic(how many quarts) and for the transfer case I need to use auto trak II(2 quarts)

Can I use 75w-90 synthetic in both the front and rear? If I have to use 80w-90 for the front differential does it matter if ti says limited slip on the bottle? And for the transfer case if I can not find the auto trak II fluid can I use something else?

So what I was thinking to use, which is offered at my local parts house is:

Valvoline 80W-90 High Performance Conventional Gear Oil for the front differential
Valvoline SynPower 75W-90 Full Synthetic Gear Oil for the rear differential
Valvoline Transfer Case Fluid for the transfer case

also, I have seen a couple different recommendations o how to fill them all. Some say to fill all of them until oil starts to come out of the fill plug and others say that for the differentials to fill about 1/2 inch from the fill plug? Which way is right?

2GCEK19N531361849.

Sounds great and thank you!

Since you looked it up what size differentials does it have?

Thank you once again, you guys rock!

When filling I just fill until a little comes out of the fill hole, right?

How do I do it?

Thanks