Lets talk about how Active Track (Super Select) works inside transfer case.
There are several key parts which allow to achieve that one-lever operation.

Input Drive Shaft 30311 goes inside front side gear 30426 and connects to pinion shaft 30423 of the center differential.
Spider gears drive Front Side Gear 30426 and Rear Output Shaft 30413.
Chain Drive Sprocket 30370 seats atop Front Side Gear on needle bearings 30306 which allows them to rotate freely relative to each other.
Differential Lock Hub 30426 seats on front most splines of Input Drive Shaft.
2-4WD Synchronizer ring 30367Q seats on the end of Front Side Gear 30426 held in place with snap ring.

Operation of Active Track is achieved by moving the t-case lever, of course, which corresponds to movement/positioning of 4WD Sliding Collar 30411.

2WD:
Sliding Collar seats in front-most position. In this position it links Differential Lock hub (as well as Input Drive Shaft) to 2-4WD Synchronizer ring . Since in this position input drive shaft is effectively locked to one of the side gears of center differential. Differential at this stage is effectively locked. Drive Sprocket is allowed to rotate freely relative to Front Side Gear in this position thus all output goes to Rear Output Shaft.

4WD with differential unlocked:
Move that lever forward and Collar slides backwards a little. Notice, there are two matching groves, one on the inside of the collar and one on the surface of the Differential Lock Hub. Also, Drive Sprocket has gear ring on the front which matches diameter of 2-4WD Synchronizer ring. Moving Collar back meshes groves and teeth on Differential Lock Hub which is now allowed to rotate freely relative to Collar. What it means is that Front Side Gear is no longer locked to Input Drive shaft and is free to rotate with different speed relative to it - center differential now is unlocked. But, moving that Collar backwards also locked gear teeth on Drive Sprocket to 4WD Synchronizer Ring and Front Side Gear as well. Thus now power is distributed between to rear and front at the mercy of differential. 4WD is engaged. Differential unlocked. 4WD detection switch at this point sends signal to ECU to disengage front solenoids and lock the free wheeling clutch.

Only in this position Viscous Coupling 30421 can show its usefulness. Since it's connected to center differential and rear output shaft, if either of axles starts to slip relative speed between Differential Case and Rear Output shaft will be different. Thus the VC will start to warm up and provide limited slip effect.
Word of caution here. If you are stuck in 4WD and start spin your tires a lot, lock the center diff. Allowing VC to spin uncontrollably for long time will ruin it. VC will possibly lock up effectively converting your truck into part-time machine. Also it will make a horrible noise by some accounts I've heard.


4WD with differential locked:
Move that lever sideways. Collar slides backwards even more. Drive Sprocket and 4WD Synchronizer ring still locked together but now teeth on the front of the Sliding Collar mesh with rear teeth on Differential lock hub again. Front Side gear is locked to Input Drive Shaft again via 4WD Synchronizer - Center Differential is locked.

Moving lever forward in 4WD Low doesn't effect this part of the t-case any more only engages Low range.

Sounds like you might have a problem with your check valve or maybe solenoids. They are located relatively high on right fender but still might get submerged, which very well could have happened when you got stuck <img src="/forums/images/graemlins/shiner.gif" alt="" /> a year or two ago. Sucking in dirt/water wont make solenoids happy. Intake manifold vacuum depends on throttle position. More throttle - less vacuum. That's why manual says to coast when trying to engage 4WD. Coasting with heavy foot off accelerator will create most vacuum in the system. Later on vacuum reservoir will hold some extra vacuum to engage/keep 4WD engaged even then car is driven hard. Malfunctioning check valve will most certainly defy that. Plenum leak won't have that much effect unless it's really bad but then you couldn't really drive your truck either.

So are you saying that when in 2WD the front hubs are NOT locked? I was under the impression that with the Super Select 4WD (Active Track) the front end was locked up at all times. If what you're saying is true, then when you shift it into 4WD (AWD) the VC engages, spinning up the front end and then locking the front hubs?

Are you referring to propeller shaft or axle shafts? Axle shafts are rotating all the time. There are no hubs on Gen 2 Montero. Right side axle disconnects inside/beside diff using air actuated free-wheeling clutch that prevents propeller shaft and R&P from rotating.

Fogive my backyard mechanic questions but you're talking a little over my head. Propeller shaft!? I thought this was a truck, not a plane! (Sorry, I couldn't resist that one.)

So, if I gather what you're saying, the axles are permenantly connected to the wheels as I suspected. But the drive shaft to the front diff, and the diff itself, do not rotate unless engaged? So when you engage 4WD the VC engages, bringing the shaft and diff up to speed, then the axles engage to the diff by some vacuum or pressure clutch system?

R&P?

AHA!

Is it like this!?

http://www.4x4wire.com/dodge/tech/solid_axle_cad/

Pretty much