Hello! If you were to crawl under the vehicle, the driveshaft should turn freely, as the transfer case has the front drivetrain disconnected in 2WD. However, while driving, the driveshaft may turn on its own, as the left axle is still attached to the carrier, which may drive the ring and pinion. Side carrier bearing failures are common on front axles and if the carrier is spinning going down the road, it can make some noise if the bearing is bad (may get louder in 4WD).
Another source of bearing noise could be the front wheel bearing. These vehicles use sealed hub assemblies and will make a roaring noise, which changes with speed when the bearing goes bad.
Another source of roaring could also be the tires. If the alignment isn't dead on, and the tires don't get rotated, the tires will "cup" and will also cause a roar noise. It doesn't take much cupping to start making noise.
Don't forget, there are two wheel bearings up front. One may be new, but that doesn't mean the other one is OK. Does the noise change pitch if you sway the vehicle back and forth (turn the wheel back and forth while driving)
Also, even though the front driveshaft was removed, doesn't mean the front axle isn't spinning. The front Axle Actuator only disengages the right axle. the left axle is always connected to the spider gears.
if you are abosultely sure its not in the front, that leaves either a rear axle bearing, or transfer case. I have been fooled before, with a rear axle bearing. I had a 4WD vehicle one time that had a bad side carrier bearing and I swore up and down it was the front axle, only to find out it was indeed in the rear axle. You have to remove the Carrier assembly from the axle to inspect those bearings.
Transfer cases have a planetary gear set, that although is not common, can fail and make similar noises. Check the fluid level and condition and see if it changes pitch or intensity in different gear ranges.