I'd look at what is in motion/use in 4wd that is not in 2wd, as a starting point.

As you seem to be implying that you have since driven in 2wd, and there was no odor, and, it is implied that you can shift into 4wd, get the odor, shift back to 2wd, and the odor goes away...the 4wd added involvement should be the key.

If the above implications are not known, or wrong...let us know, or, take it out and go back and forth between 2wd/4wd enough times to be sure.