Are you sure you weren't in Hi rear? Gravity could have pull the rear shift linkage down while you were "free wheeling". That's why they say to never park a 2-speer rear in Hi range (something bumps the truck and the next thing you know it's at the bottom of the hill).
If you are positive you were on Low rear you must have hit a pot hole while free wheeling that jumper the unloaded rear out of gear.
That's my thoughts...
EDIT, I was typing while Jerry posted his thoughts...I think we are saying the same thing...
Mike B
