Sounds like bearing in the front of my 1500 chev in four wheel only lound whine

If it whines when in the 4x4 position but not at any other time then the noise you hear may be coming from the transfer case and is caused by the high gear load of driving both axles on a this type of surface. If you are driving in the 4x4 position on any dry hard surface you will hear some noise from the transfer case and usually it is a noticeable whine. Also, driving in 4x4 on a dry/hard surfaces greatly adds to the strain and wear on the power train. It would also be a good idea to have the both axles and transfer case fluid levels checked to be sure none of them are low. Hope this helps a bit and good luck!