Sounds like the box is stuffed then, allthough you might want to check the diffs, jack up the back of the car and turn the prop with the box in neutral and see if the shaft turns without grinding, then do the same for the front, if they both seem fine then theres summat up in the box or transfer box.
Does this happen in both 2wd and 4wd?
The 6cyl box is probably a bad idea, from what I know the box is the same except the bellhousing, but the housing is also the mounts for the shaft bearings and other bits in the 'box, it'd be a major rebuild to swap the bellhousing, allthough you might be able to use bits from the 6 cyl box to fix yours.
This any help?