Hi there, if I understand correctly, you can shift from 2-high to 4-high just fine, but when you try to go to 4-low it grinds. Is your jeep an automatic? If so, before you shift to 4-low, I think you should be in Park or Neutral so your tranny isn't trying to spin the input shaft to the transfer case.
2H & 4H use the same gears in the transfer case, you're just engaging the linkage to the front end. When you go to 4-low, however, the transfer case has to pass through its own neutral (not the one in the transmission) which would allow the output shaft from the tranny to spin and cause the gears in the transfer case to grind. Whenever you go to 4-low, make sure you are stopped and the tranny is out of gear.
Hope this is what you're after...
