That is very similar to the problem I had on my F250. It is the long bed with extended cab and has a two piece drive shaft with carrier bearing. The carrier was going bad and I didnt know it and it wore out the U joint on the shaft and it came apart while I was pulling a hill with my travel trailer. That was a pisser. But 4 wheel drive is good to have at a time like that.

Glad you found it.

MikeR