4WD front end noise on coast

2000 Model Chevy Silverado 4x4. 135000 miles. 4.8L v8. On coasting the vehicle, I am noticing a whining noise that becomes more like a gear gnashing as the ground speed drops from higher to lower. Could this be caused by front hubs not unlocking? T-case is manual. Could the linkage be loose? Seems like the focal point of the noise is under the front end of the truck, slightly off center to the right. Could this simply be low fluid (Haven't checked it yet) or is this something more complex? I recently replaced the rear differential gears with the same ratio. Any help would be greatly appreciated!
Left a small thank you with you guys. Hope it helps a little.