4wd light turns on when driving in 2wd?

Hi all, I have a 2006 STX D40 Navara, and only recently the 4wd indicator beside the clock turns off all together and the 4wd light on the dash turns on when driving on the highway. Its not in 4wd when it happens and doesnt go into 4wd unless i stop, turn the knob to 4wd and then it will go into 4wd. Then when i turn the knob back to 2wd all the dash lights go back to normal. It has happened about 4-5 times in the last 2 weeks. Does anybody have any ideas on what could be going on or how i can fix this?
Thanks.

As far as I recall the 4WD light was an indicator of a fault. Let me go get my manual and see what I can find out about it.

The manual (p5-16) confirms that the "4WD" indicator light indicates a problem that you need to attend to.

* If it blinks, it means the difference in wheel rotation is large (the radius of the front wheels is too different to the rear wheels).

* If it's solid, it means there is a malfunction in the 4WD system.

With the light on, you might engage 4WD mode, but the car will not actually obey (this is according to the manual) and remain in 2WD mode. It should indicate this in the 2WD/4WD indicator at the bottom of the display near the clock.

Rock
normally when the light comes on it as old tony said there is a fualt with the system

i would have a look at your 4wd low switch its located on the left hand side of your transfer its the one closest to the back they normally test ok with a multi meter but as soon as you put them back in the car they shit them selves and the other one to look at is the wait detection switch its located at the back of the transfere case
normally i replace both at the same time and you have no further problems.
i have had a lot of navs do the same thing as what you are saying and its always them pair of switches
hope this helps

My 07 STX had the same issue, mine ended up staying on constantly took the dealer an eternity to sort it, it ended up at an Auto Elec for two weeks, in the end all sensors/switches were replaced on the transfer case. That was around 30,000km ago so far so good..

Thanks very much guys, that helps heaps. Ill check those things out and let you know how i go.