Originally Posted by SolarColumbia
Are you getting any vibration? If so could be the front driveshaft u joint.
To confirm/test this, take your front driveshaft off and drive it around and see if the sound persists. Although bad u joints tend to rattle or squeal. I also doubt it would be the front because when the gas is applied in 2wd the front driveshaft doesnt do anything.
My first guess would be your throwout bearing. Second guess would be differential. Third would be maybe your clutch isn't disengaging all the way or sticking a bit.