How to Shift into 4 Wheel Drive
Slow roll between 1 and 5 mph in neutral. Also shift quickly, don't drag it out. Some Jeeps engage better while slightly moving, and others lock in while stopped.
"With the vehicle rolling at 2 to 3 mph (3 to 5 km/h), shift an automatic transmission to Neutral or depress the clutch pedal on a manual transmission. While the vehicle is coasting at to 3 mph (3 to 5 km/h), shift the transfer case lever firmly through Neutral and into the low-range position."
"Shifting into or out of 4WD low-range is possible with the vehicle completely stopped, however, difficulty may occur due to the teeth of the gears not being properly aligned. The preferred method to engage the low-range is to slow down to 2 to 3 mph (3 to 5 km/h) and put the transmission in Neutral. While still moving forward, move the shift lever firmly into the four-low position. Then return the transmission to the desired gear. Refer to your owner's manual for detailed instructions."