2003 Yukon "AWD" with Stabilitrak

[sagent]

I just pickd up a 2003 Yukon with the AWD / Stabilitrak option. When driving the truck home, usually when letting off the gas from abt 80 mph, I would get a low pitched drone / resonation accompanied by a slight vibration. It is almost as if someone turned on a subwoofer in the truck with a constant 60 hz signal to it . It would last for abt 5-15 seconds and then go away. The DIC indicates all is well with stability system and no warning lights on at all. I got the truck home and checked the tire pressure in the morning all the tires were abt 8-10 psi low. I have not had a chance to take back on the freeway yet to check it out but has anyone else had any of the same type of issues and what did it take to get it fixed? Any ideas ? Truck has 95,000 miles. This noise occurs only occasionally.

 

Also appears reading the owners manual I should use Dexron III in the transfer case. Is this true?