2012 Yukon XL Denali fluids and such

[MASTERBrian]

I picked up a 2012 Yukon XL Denali last October. She has about 80k miles and I'm wanting to go through the fluids, just to update them and insure everything is up to spec. I know I'll be ahead per the book on a few of them, but I have no clue how the previous owner treated everything and so forth, though the truck does have good service records for oil and such.

That said, I have 2 qts of BG Products SAE 75W-90 Full Synthetic Gear lube. I plan to use that until I'm out of it, I'm not sure how much the front or rear diff take. I've used and liked Royal Purple in the past and figure I'll top off with that. Any issue with either of those for this? I think I saw the front had a drain plug, I'm not certain. If not, do I buy a gasket to close back up or just RTV it? Same question on the rear? I've been told on my 2003 burb some like the gaskets some say just RTV.

The Transfer Case says Dexron VI Auto Trans fluid in the manual....this is the All Wheel Drive. Does that sound correct? I think it does have a drain plug and I actually worry there is a whine coming from here. Any idea what that might be? I plan on taking her to the Transmission shop for a filter and fluid change. I know this is not called for until 156,000 miles, but I feel these are weak points in the GMC/Chevy product. Every single one I've owned has needed these items replaced by 180k-200k miles. No clue what the maintenance was on them.

Any tips beyond this??