99 kenworth w900 won't move unless power divider engaged.

Not likely. Then the power divider would be shot.

 

There is one other situation where I saw a similar condition which didn't involve the differential gears being shot... when I ran a service truck, we had a driver come in (into the shop, this wasn't a road call) with the same complaint.. turns out he somehow managed to snap all the lugs on one of the axle shafts on the #2 axle, so the shaft was just spinning freely. Would love to know how he managed to do that.

 

That sounds about right. Earlier in the winter, we were moving trailers and trains to clear the snow and one of our drivers had this same problem (on a 04 Pete) and it had to be locked in order to move. He didn't know what he did, but my guess would be hit the switch when the wheels were spinning. I never heard what the shop found though. Probably the diff. I've seen a couple of our trucks in the shop this winter getting whole new diffs put on.

I'm not sure what's so hard about hitting the switch at the right time.

 

Any time I roll off the pavement onto uneven ground I lock in the inter axle. Only takes one wheel off the ground to spin you out when you're unlocked.

 

Well, he's not the first to do that. That thru shaft will break.

 

Drivers will panic and hit the power divider switch too fast when they encounter a wheel-spin situation. You have to consciously train yourself not to do that because it's somewhat of a natural reflex to hit that button right away before you think twice. One of the big reasons why 6x2 setups are gaining popularity. More idiot proof.

On another note I remember this late 60's International Cargostar garbage truck I used to occasionally drive for the furniture company I worked for in college. Somebody broke one of the floating axles one day and it quit moving for awhile. I honestly can't remember if I did that or not but I remember that think was a total death trap.