If the auto hubs are working fine (they are the most likely suspect), and the front prop won't move with the transfur in 4x4 (car stationary), then the other options are exploded diff or brocken drive / half shaft. It is an F73? Indipendent fron suspention? Did they call the F70 van a Fieldman? Anyway, if it's an Indi, try the jacking up thing again, but this time turn the drive shaft rather than the wheel. This will mean the auto hubs are not involved. If it is in 4x4 (and the front prop won't turn) then turning one driveshaft should couse the other to turn in the opposite dirrection. If this happens the prob is the auto hubs, a brocken universal joint on one of the driveshafts or the splines have been striped off on of the driveshafts inside the hub. If the other driveshaft doesn't turn then it is a brocken diff, the splines striped off the diff end of one drive shaft or a driveshaft sheared off inside the diff casing.
One more thought. Are you sure the front prop is not turning? It could be that the transfur end is sitting still, but the diff end is spinning if the splines have been stripped off the slidder in the middle of the prop. not likely, but poss.
I would still suspect Auto hubs as most likely culpret...

Any veiws expresed in this thread by me are purely from my own experience, and (sometimes) falible memory. Hope my comments help, but please don't take them as gospel.