Whatever it is, it's most likely internal to the transfer case, which makes me think the replacement you put in is defective. Why did you change it in the first place?
Assuming that we're discussing a second gen vehicle (that's the forum section you posted in), the early transfer cases had a problem jumping from 4L to N and two of the internal parts were redesigned, I believe a gear and possibly the selector hub, this might be related to the problem you're having, if the replacement case cam from an early vehicle, or it could just be worn parts.