Originally Posted by X-Traction

My guess is that the truck has automatic locking front hubs, which have to rotate up to one revolution before they engage. So if you weren't moving, you were left with 2wd.

This is why some choose to install manual hubs. If you get stuck without the automatic hubs engaged, you may not be able to move enough to get them engaged. This has never happened to me, but I can see how it would happen.

Next, I gather you either don't have a limited slip rear differential, or if you do, it needs servicing. So you sat there with one wheel spinning.

This is why limited slip rear differentials are so useful. Had you gotten the front hubs engaged, you might still have been stuck if, say, you were tilted uphill. You could then spin both tires on the passenger side. A limited slip rear diff. would also solve that problem.

The behaviour of 4wd systems can sometimes be quite puzzling, and hopefully someone will correct me if my comments above are wrong.