4WD whine
 

This morning on my way to work I was using the 4WD since the roads were slick with snow and slush.I noticed a whine that wasnt there the last I used the 4WD.The whine comes in around 35-40mph and quiets down when I give it power.Is this a sound that normally appears after a while?Its a 02 Silverado 2500HD 6.0 Automatic with the 4WD on the floor shift 100K.
I have not had the chance yet to check the fluid level.Where do I check the transfer case level and if low do I need to use the GM fluid or is there a suitable after market fluid I can use?
Thanks.