My 2000 xterra is stuck in 4wd...can't get it out.

There are some ideas in the link.

https://www.google.com/search?q=2002+nissan+xterra+clicking+noise+in+4wd

JACK ALL 4 WHEELES OFF THE GROUND START IT STAY IN THE DRIVER SEAT HAVE A FRIEND SEE WHAT WHEELS ARE TURNING THEN WITH FOOT ON THE BRAKE PEDAL PUT THE SHIFTER AND FOOT ON THE BRAKE TRY TO GO INTO 4 LOW LOOK TO SEE IF THE DRIVE SHAFT IS TURNING IF IT IS AND THE WHEELS ARE NOT CHECK THE HUBS ECT

switch at shifter linkage is out of adjustment

Yes check the fluids, check the 4WD lever linkage and see if its not binding/bent or stuck with mud.

first put the van in neutral while in neutral try moving the shift

Pull # 1 spark plug, install compression gauge or somehow block hole and jog engine until you feel or record compression, then line up TDC marks on crank pulley. Now when you drop distributor in, rotor should line up with # 1 spark plug wire tab under cap. Make sure you have all ignition spark disconnected, one way or another. If you haven't pulled other distributor yet, it should be easier, just mark housing for reference, jog until rotor is at # 1 tab, remove old one and re-install new in same position.

When was the last time you shifted to 4WD?

If you do not use 4wd too often then try this old trick.

Next time you're going off road try to do the shift to 4WD,shift to reverse and drive in reverse for 6-12 feet.
(no gas //1 mph //SLOWLY)

Driving backward for few feet help the gear pickup the lubrication and let the gear sync up better.

Check your Xterra gearbox lube schedule.

From the service manual:

All you need to do is unbolt the front brake calipers and the rotors just slide on and off.