The following recommendations for shifting into 4 wheel drive were taken from post #95 of the Yahoo Nissan Xterra Solutions club site. The information in quotation marks is by Yahoo member ID, FilfilGuy who himself is quoting Barnstormer from the Unofficial Nissan Xterra Board. What follows the quoted material is my own two cents worth.
"To shift from 2H to 4H, you can be moving, though you shouldn't be driving faster than 25 MPH. It is not necessary to engage the clutch, though it might help make the shift easier.
To go from 4H to 4L, the vehicle must be stationary. Shift to neutral, then push the transfer case lever to the floor and back all the way to 4L. To shift back out of 4L, reverse the process.
The back of your sun visor has a concise list of instructions. If you have an automatic, take a look at the following that was posted on the Unofficial Message Board by "barnstormer"
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The manual doesn't explain the procedure properly for the automatic. The visor instruction don't make sense either.
Shift fast is part of it... but the trick is...
When shifting between 4Lo and 4Hi be in neutral or park with your foot fully depressing the brake.
Then with the palm of your hand push the shifter straight down into the floor.
Once the shifter is fully depressed into the mechanism pull the sift lever way back fast until you feel and hear the gears catch.
Note that you don't have to press the shifter down when shifting between two wheel drive and 4Hi only between 4Hi and 4Lo.
Shifting on-the-fly between 2 wheel and 4Hi also has some problems.
Shifting on-the-fly with auto between 2 wheel drive and 4Hi.
Never do it on a banked turn...only on straight away. You must be going straight for about ten seconds on level grade. Works
best if stopped.
Here we go.
Slow down to below 20 mph in a place that you can pull over if you have to.
Without pushing down ... pull shift lever back fast to the 4Hi position. You shouldn't hear gear grinding...but should feel the
4Hi catch and kick in.
You should never find any problem shifting from 4Hi back to 2 wheel drive. Although you should note that the change
doesn't happen immediately. The light will change between 6 and 15 second after you have shifted back into 2 wheel dive.
You also won't feel anything happen.
Good luck and enjoy the extra hauling power of the Automatic Transmission Xterra.
That was an excellent description of how to shift into 4 wheel drive, Kevin (post #95). You should help Nissan write their next version of the owners manual."
I've had trouble shifting into 4 wheel drive too, FilfilGuy. If you don't shift just like you said, sometimes you get stuck in the neutral area of the transfer case between 4L and 4H and the shifter grinds gears, which ever direction you try to go. If you turn the engine off when this happens, you can complete the shift without grinding. Start the engine again and drive away.
I started off with nothing and still have some left.