I was incorrect if I suggested that the default state of the rear final drive/electronic controlled coupling is locked. My coupling was/is seized. It wasn't until I spotted an older post that I finally pieced some info together that others might find useful because it isn't an easy problem to be aware of nor diagnose. My "project" vehicle has rarely been driven any distance because it has needed so much work to pass provincial inspection. I had hoped that the replacement of the rear suspension member ("subframe") would be the end but now I'm deciding how to solve the seized coupling. I did decide to try to disassemble it. I had nothing to lose, and began by cutting the welding tacks on the output side of the housing. But from my experience and from what I can glean from the only schematic I have ever seen of the part (in service manuals), it seems clear that a seized electronically controlled coupling simply cannot be taken apart. I had the vehicle on the highway briefly to take it to a glass place to get the windshield replaced (the classic crack that starts in the lower corner, left in my case, goes up at about 45 degrees and over about halfway across; this is caused by a small spot of rust that expands and stresses the windshield in the lower corner). On the highway if I went up to about 105 km/hr the front end started to severely shake, like a badly out of balance tire or an out-of-round tire or rim. The old post from here from a while ago described a similar problem. The poster tried a number of things to figure out the problem and repair it but eventually gave up and sold the vehicle. Now with the rear end locked in 4WD you shouldn't go faster than about 30 km/hr (ca. 19 mi/hr) and if you do the ECM will normally override the control and force the electronically controlled coupling into 'AUTO', so AWD, mode. I don't know exactly what is happening but if you have a seized coupling and get up over 100 km/hr apparently you get that extreme shaking; it's safe to assume that isn't good for one or more parts of the drivetrain. Another point that I guess somewhat surprised me is that the rear final drive in AWD/4WD T30s was used in other Nissans, including the T31s, but also in the Rogues up to about 2013 (which were the same platform as T31s internationally anyway) and the J10 Qashqai (originally the Dualis) of the same general era. What I am not sure about is if the other rear ends are actually the same as the T30 XTrail because the Rogues with that part have a 5.173 final ratio (the used ones on ebay claim that at least) and I don't think that is what the XTrail uses. I have not been able to find out what the spec for the T30 is and I have looked through service manuals, users manual and the Internet and the best I can find is on the detailed sticker on the door jam which lists the axle as an FT40, whatever that means.I now have important new insight into the rear final drive ("differential") in AWD/4WD T30 XTrails because I have just removed the rear subframe (Nissan calls it the "rear suspension member") because it has corrosion severe enough that it failed our provincial inspection. The rear final drive is entirely supported by the rear subframe and so it must be disconnected from the subframe and lowered down enough to permit the subframe to be dropped down enough to be removed. What I discovered while doing this rather unpleasant work is that the default state of the rear final drive (so with the ignition and obviously engine off) is engaged, not disengaged as I have assumed. The rear final drive state is performed by what Nissan calls an 'electronically controlled coupling' and so when the car is actually started in 2wd the coupling disengages the rear axles. However, while disconnecting the rear final drive from the propeller shaft, so removing the four nuts, then the bolts, I wanted to rotate the propeller shaft to better access each nut. The front wheels were on the ground and I turned the ignition to the point where I could shift the transmission into neutral but that didn't allow me to rotate the propeller shaft until I raised one of the front wheels with a small floor jack until the tire just lifted off the driveway. Then I could rotate the propeller shaft either by turning the front wheel and in fact at the propeller shaft itself. Then it finally dawned on me that in 2wd the propeller shaft cannot simply rotate down freely to the rear final drive because otherwise the front wheels wouldn't be driven. The T30 Xtrail service manual, as far as I can determine, does not actually explain exactly how the electronically controlled coupling in the rear final drive works. It clearly will lock the rear axles in when in 4wd, and at times when in AWD but it must actually lock the propeller shaft when in 2wd. So that means you cannot simply remove the propeller shaft at the front and make the vehicle permanently 2wd and it means you cannot diagnose possible center bearing problems by disconnecting the propeller shaft at the rear final drive input as I suggested as a possibility in my note.