My 98 Dakota has 173k miles on it and is making a whining noise that starts at about 30mph in 2wd. Up until then it's very quiet. Doesn't change when I put it in 4WD. It has the 231-D transfer case. I bought another one with just about 90k miles on it and we swapped them out but it still whines. The dealer told me that it was the transfer case but now I'm beginning to doubt what they said. I have serviced the tranny and front differential as well as putting fresh fluid in the newer transfer case. All u-joints look fine. HELP?

I neglected to mention that when I put the truck in neutral while driving the noise goes away.

 

Welcome to DF
If the noise sound like it coming from under the driver seat. Replace the large output bearing in the overdrive unit. I'll move your post to the 2nd Gen Dakota section. This way you will more response there.