Transfer case difficult to shift into desired range
1. Speed may be too great to permit engagement. Stop the vehicle and shift into the desired range.
2. Shift linkage loose, bent or binding. Check the linkage for damage or wear and replace or lubricate as necessary
3. If the vehicle has been driven on a paved surface for some time, the driveline torque can make shifting difficult. Stop and shift into 2-wheel drive on paved or hard surfaces.
4. Insufficient or incorrect grade of lubricant. Drain and refill the transfer case with the specified lubricant.
5. Worn or damaged internal components. Disassembly and overhaul of the transfer case may be necessary

Transfer case noisy in all gears
Insufficient or incorrect grade of lubricant. Drain and refill

Transfer case noisy or jumps out of 4x4 low range
1. Transfer case not fully engaged. Stop the vehicle, shift into Neutral and then engage 4L.
2. Shift linkage loose, worn or binding. Tighten, repair or lubricate linkage as necessary.
3. Shift fork cracked, inserts worn or fork binding on the rail. Disassemble and repair as necessary

4. Inspect bushings on linkage and replace as necessary.

Lubricant leaks from vent or output shaft seals
1. Transfer case is overfilled. Drain to the proper level
2. Vent is clogged or jammed closed. Clear or replace the vent.  If using extended vent tubes, make sure the tube isn't sagging and allowing fluid to collect and not drain back in to the case.
3. Output shaft seal incorrectly installed or damaged. Replace the seal and check contact surfaces for nicks and scoring.