I changed the transfer oil on my 2001 4Runner with 75w90 dino. While driving after I put it in AWD and I swear I hear a whiring sound that I didn't have before. I am not sure since I rarely use AWD and totally unclear how it icould be related to the transfer case oil change. Any ideas? It has 65 K and case was changed by dealer at 20 and 40 K. The only other thing I tried to do was change the front diff. This failed since the drain plug is apparently welded on. I did some serious pulling on it before quiting

 

Double check the fluid level and the type of fluid you put in there. There's not much that can go wrong with a fluid change if the level and fluid type are correct. I just changed all the drivetrain fluids in my 2000 this past weekend too, I used Mobil1 75W-90 Synthetic gear oil in both diffs and xfer case.