99 drivetrain noise/declining MPG

[TRE-X]

I currently own a 99 gmc 1500 that has turned out to be the biggest piece of junk i have ever purchased. From the time i bought the truck it had the squeak from where the drive shaft goes into the transfer case and was told it was normal. At 45K miles i replaced the rear brakes. At 52K miles the ABS module went out. At 55K i had to clean the throttle body because the gas pedal would stick. At 62K the fuel pressure regulator went out, and now at 66K i replaced the frond brakes and the carrier bearings in the front differential. Sorry about that but i just had to vent.
The current problem is that when driving at highway speeds (45 and above) and the truck shifts into OD there is a howling noise that comes from under the truck and you can feel it through the gas pedal. It is the worst when the truck is at 45 MPH and you just barely hit the gas. It feels like the truck is dragging. The problem amplifies when it is in 4WD and the truck is under load. i also noticed that my MPG dropped from 17 to 12. The GM dealer looked at it and said it was the front differential bearings so i replaced them and the pinion bearings but the problem is still there. I am sick of spending more time under my truck than driving it. I also put 80W-90 in the front differential instead of the synthetic because i heard that it will reduce the noise but also reduce MPG. I had a 89 chevy with 325,000 miles on it and never had this problem and the front differential is the same unit. Any info. would be appreciated.