We have a 2001 Trooper. The 4wd is just a simple push button on the dash. This winter it stopped working. On the display, it flashes like it is trying to switch but never fully switches over. The ABS light is on all of the time (don't know what that means or if it has any connection to this issue) and if I repeatedly try to get it into 4wd a big read error sign lights up. I love my Trooper but it is pretty useless in Wisconsin without 4wd. HELP!

SOURCE: 1999 isuzu rodeo 4wd not woking

I had this happen to a 1999 4x4 LS and my sisters boyfriend mentioned to periodically turn 4x4 on/off at least once a month.
What I did was go down a stretch of road slowly and attempt to turn it on/off till it became unstuck.
Drove around 10 miles on a road that was hardly used...to get the stupid light to quit flashing I had to put it in park and try to creep forward ---while using off settiing or put in park then reverse...while using the off -setting on 4x4. My patience paid off. They get sticky...you must be cautious... because there is a way to cause vibration and then it won't start...some deadman feature that I experienced once and the LWD must have slipped in a little and I had to look at everything.
Mine is an Automatic with the shift on the floor...that can get sticky too.

Good luck and hope this helps...mine had lots of growing pains but I get 23 mpg and it runs/idles better and the only gremlin left is some short in the dash which leaves the lights D/3 (one or both on)for the shift indicator on:)

SOURCE: push button 4wd not working

It sounds like someone has changed into 4wd at speed and broken the internals of the system. Check the relays and fuses first but it sounds more serious than that. If it drives in 2wd put it in reverse an circle listen for any undue noises.

SOURCE: 2001 Trooper: Front axle will not engage when I press dash 4x4 switch

hi guys i would check all the small vacume pipes that conect the sol valves toi the vacume actuators having just replaced the engen in mine i found that these do not have any clips holiding them on tight and can get oil on them and become oversized and leak you can test the diaframes in the actuators by putting a length of pipe on one port and sucking by mouth you can hear the actuator move and then find you can put toung on end of pipe and it will hold a vacume if it does not then chances are the actuator diafram id u/s hope this helps bill

SOURCE: How to read the DTC´s from my 1992 Bighorn PCM (computer)

I found a free download, workshop manual online. It is 3500+ page PDF that includes every diagram and all info that you could possibly need. I dont remember the exact website but if you google iszu trooper pdf free download etc. you should have no trouble locating it.

SOURCE: I have a 99 isuzu trooper (jackaroo) 3.5 liter 4wd

sounds like a sensor - when did it start - if just cranked up one day and bam it is there - sounds like either a brake wear indicator issue - if they are connected in those - or a fluid level change in the rear end or transfer case - look for leaks and check the brake pads or shoes