Rear driveshaft not spinning, truck only drives with power diff locked

I respectfully disagree. Think of a broken axle shaft as a spun out wheel on ice. All power will be transfered to the spinning wheel (the broken shaft). Once you lock in the power divider you will allow power to transfer straight through to the rear drive axle so that it will provide drive.

Here is the power path with the power divider both locked and unlocked straight from Eaton:

 

The easiest way to isolate your problem is to.
Jack up a wheel, Chock truck, release brakes & put in Neutral. Then turn the Wheel, the Drive shaft (or if It's a rear wheel the through shaft) should turn.
Repeat process for all 4 wheels.
This will tell you if you have a Broken Axle, or which pumpkin to pull.

Ask me how I know this.....LOL..

 

He can still have a blown front axle. When the power divider is locked, it puts power to one front drive tire and one rear drive one. Doesnt matter if the front drive one is broken or not, it will still power the rear one.

 

That diagram you provided just proved what I just said.. and nothing I didn't know already.

You are correct in saying power always goes the path of least resistance. It's going right to that snapped output shaft on front rear end. Now then, when you lock the divider the front rearend has to turn. This is why he made it home.
If he had a broken axle on the front rearend he wouldn't go anywhere.......

Anyway I'm feel like I'm beating a dead horse here. Like others have said already, your gonna have to pull it apart anyway, so he will know soon enough.

 

Read his post . His jack shaft is not turning but his front driveshaft is!!!
Holy.

 

@swaan here is what the OP wrote:

I interpret that as the inter axle driveshaft is not turning when he unlocked the power divider. If it was the other way around and not turning while locked in I 100% agree with what you're saying.

 

And??

Front drive shaft spins, interaxle doesnt.
If he had a broken front axle, power goes down the driveshaft and then to the broken front axle. Truck doesnt move, interaxle doesnt turn.

If he then locks the power divider that means 1 axle on the front must turn and one axle on the rear must turn. 1 axle on the front will turn, the interaxle will turn, and 1 on the rear must turn. It does not matter if both of the front axles are broken, a rear axle must turn as well which can power the truck!!

 

When you say....jack shaft.... is that the short drive shaft between axles.....inter-axle driveshaft?

I've had this happen couple years ago.i replaced with rebuilt pumpkin was about $2100 for parts. Didn't have to concern myself with metal filings in old differential.

 

I'm trying to learn..
Because the inter-axle doesn't spin the damage has to be in front rear end. Not an axle shaft in front because when diff locked, the axle spins. Broken axle, no go.
Correct?

 

I'd put money on a broken output shaft from the front axle. Or a stripped input shaft to the spider gear assembly in the inter axle differential. If anything was damaged in the rear axle, the inter-axle shaft would be spinning freely.

Looking at the service manual and when the power divider is engaged, the sliding clutch directly connects the input shaft to the forward differential pinion shaft via the side gears.