Hello Everybody,
I just got my montero back from the shop after the engine rebuild and have been changing the fluids.
The rear locker motor would run forever so after looking through Service Manual, decided to find out what is wrong.
I diconeceted the air hose while turning on the rear locker (in parking) to see if it is producing any pressure, I do not have a gauge but by blocking the end of the hose with my thumb I feel a bit of a pressure which I assume would be around 5 or 4 psi as the service manual calls for.... next step I took out the locker switch from the rear differential to check for continuity, it seemed to have oil in it and did not have good continuity, cleaned it with carb cleaner and that seemed to help, but you had to pull really hard on the switch to achieve that. After that I installed it back, went for a drive and noticed a weird winning from the rear differential and difficulty in maneuverability , brought it back home, changed the fluid with new one (80w90), went out for a test drive, rear diff is still locked, tried driving in 4hlc and 4llc to see if that would release the lock but no help, AND 4H does not engage while gear selector in D. I do not have the owners manual to see what I can do? any advise on what I can do would be greatly appreciated.
(rear diff motor still runs forever)

Possibly the pressure relief valve in the pump is stuck? Have you disconnected the air line and then drove to see if it would unlock? The unlock position is spring loaded so it should unlock itself with no air pressure applied. Report back.

Just disconnected the hose drive in for a few feet, in the driveway, still looks locked, and can not engage 4H while in drive.

disconnect the plug to the pump, disconnect the air line, and back it up about 12' and see if it unlocks.

There is a logic box in the system when stock. It will run the locker pump when in low range only. Either the logic box or the locker switch is bonky, OR, the engine rebuilder/installer pooched some wiring. I don't know if there is a defeat on the shifter to keep you out of 4hi when locked up, perhaps another function of the logic box, but I believe the tcase shifting is all mechinical.

It's possible some of the tcase gear position switches are misconnected, confusing the logic box.

Disconnected the air hose again and drove for a couple of yard including both hand turns and reverse, I discovered that 4H does get engaged while in D but looks like rear diff being locked sometimes prevents it from doing so.
Locker switch ONLY engages when in 4HLC/4LLC while in Parking or Neutral, and everything was working good until I trying messing with the locker today as mentioned in original post. I am assuming no electrical issue but a stuck locker??? is it possible that the actuator is not coming back to its normal none compressed stage? would applying some sort of vacuum help it decompress?

Using a little 12V matter air pumps, used the "vacuuming" the pipe to the differential, drove the car up and down the driveway, still lock <img src="/forums/images/graemlins/frown.gif" alt="" />. Help a new fellow Montero owner here guys is it possible that the switch is holding the actuator?

Looks like I might have to take out the differential carrier (as noted on the Service Manual). apart from the outer bolts are there any other surprises inside I need to be careful of?

Put both rear wheels in the air, and turn one side. If the other side turns opposite, it's unlocked. If it turns the same direction, it's locked. Whang the housing with a bfh and check again. Rattle the wheels back and forth. Repeat as necessary. If no relief, pull the axle shafts and pull the diff chunk and inspect the locker actuator.

Thank you Fastteddy, that looks like an awesome way to know if it is locked, as said I am new and every single bit helps. I will do that today after work and report back my finding. <img src="/forums/images/graemlins/cheers.gif" alt="" />

For future reference it's also how to check if a vehicle has a limited slip differential.