Even though 4x4 is not engaged, the driveflanges keep everything spinning from the front wheels back to the transfer case. So your problem is somewhere between there.
There was a TSB for the 96-97 Pathfinders on 30-40mph vibrations being caused by the front driveshaft not being reinstalled corrrectly. Originally from the factory there was a blue mark on the driveshaft that should line up with a yellow or white mark on the companion flange of the front differential. But the marks fade over time.
If the marks are gone the TSB had you install the front driveshaft and if there were vibrations, rotate the propeller shaft 90 degrees in relation to the front differential flange and road test again. Basically trial and error to see which position produces the least vibration.
1997 Pathfinder XE
Air Lift Air Bags, Warn Hubs