Yes the truck will roll in park with the center diff unlocked and something like the front or rear driveshaft removed. Ask me how I know, I about ran myself over.

I would imagine the same thing would happen with a broken cv or rear axle shaft as well.

Dang, that's scary. How bad were you hurt?

So I am trying to understand how the AWD system works; under normal conditions, if only one wheel was spinning (no traction), none of the other wheels would get power from the TC unless it is locked?

Well I was behind the vehicle and lowered it down with the floor jack onto the tires and it rolled back a foot towards me until I through a plastic tire chock at one of the tires and it stopped.

Correct its just like any normal rwd vehicle except it can choose to send power to any of the 4 wheels when unlocked instead of just 2.

I am not sure if this would make sense, but I went outside to see if I could tell if either one of the CVs was bad. I used my phone to record each side to see if the CVs would rotate when I applied power. If they rotated while I was applying power that should indicate that CV was bad correct?

Well, neither one moved. However, I did not notice that the front driveshaft was spinning when I accelerated. Could it be a bad front diff? If none of the wheels are getting power but I can clearly see the front diff spinning under acceleration, that would make me think the front diff is the problem?

Did you jack the front tires off the ground to test this?

I have not. Should both sides be off the ground at the same time? Would I be looking for one of the wheels to spin?

Given that I see the front driveshaft spinning when I accelerate, without any power going to any of the wheels, the problem definitely seems to be up front, and not the TC correct?

Oh, nevermind, I get what you were trying to do now.

I'm not sure if this helps but this is my experience.

I grenaded the rear ring and pinion on a trail in moab and limped it off the trail. Whenever a R&P breaks it drives fine for about an entire tire rotation until it catches, or looses mesh on the broken part of the teeth, and you can feel a huge shudder/metal grinding noise until the teeth mesh up again. This keeps your speed to a max of 3 mph with a large 'hiccup' every couple feet. I did manage to hear gear grinding noises (like what you said) shifting maybe to park, or out of park, I don't remember, when loading onto the trailer.

But I don't know of any other failure mode of differentials other than a destroyed R&P which you would have definitely noticed.

There is your answer. Your V8 has full time 4WD so there is a variable split of power to the front and rear drive shafts when the Torsen center differential is unlocked. Since the differential is unlocked, power will always go to the path of least resistance and in this case, sounds like something is broken in the front differential allowing the drive shaft to spin freely without resistance. Given that the Torsen center differential is essentially a limited slip, I wonder if you get any vibration or rumbling when your front drive shaft is spinning freely as the diff should be trying to "lock up" and send power to the rear.
Regardless, you say when you lock the differential you can get the truck moving, that's because you have a physical lock that forces a 50/50 power split and your rear driveshaft is being forced to spin alongside the front driveshaft (ie. Power is no longer going to the path of least resistance).

Hope I did a decent job of explaining what's happening. This definitely sounds like a front differential issue.

That does help and it makes me really think it is the front diff that is bad. Given that when my 4Runner rolled off the trailer, and started to drive normally, the front diff may have spun enough to catch on the part of the ring that had teeth, to drive around. And then once it spun around and got to the bad part, the transfer case just sent all the power to the front (least resistance) and the front diff could not engage because the ring is jacked up.

That would explain why I see the front driveshaft spinning with no power to any wheels. And then why locking the center, I was able to move, because half of the power was sent to the rear.

Thanks for the help everyone. Hopefully the front diff is the issue. I am taking it back today.

You did! I did not see this before I posted my last one.

I definitely am getting some rumbling when I put it into drive. I hope a new diff fixes it.

Off topic question - assuming that I get a replacement front diff, I should be able to rebuild the bad one myself and have a spare?

Thanks!

You said you drove 70 miles on it correct? The R&P would be fine if you did that.

Yes I did. It was mostly on the freeway where I was constantly moving. It was only when I exited, and came to a stop, when there was no power getting to any of the wheels.

So, it still could be the CVs? I had replaced those about 2 years ago from AdvanceAuto; maybe they're junk?

I'm still guessing t-case. It being stuck between locked/unlocked or something. Not sure. Good luck.

Just to check, if on my old TC I can do a full spin on the input shaft and see both flanges turning, would that be a good sign the TC is ok or could there be something else inside causing it spin without engaging anything?

I had considered that too, but I do not know exactly how these TCs run.

Maybe I should just do a FJ t-case/front diff conversion!

Not sure, maybe? Try changing the oil and see if any metal comes out would be another easy check.