1/8 diff

[Patrick]

So the center dogbone is turning, and the outer ones aren't?
If the diff pinion (input) is turning, but the outputs aren't (and the yokes aren't slipping) then it could be spider gears stripped, crown and/or pinion stripped, screws that hold the crown gear to the diff cup have broken, the pin that holds the sun gear (I think, the ones that are driven by the spider gears and are on the out put shafts) to the out put shaft could be broken.
Has it made any crunching noises? Usually when teeth strip, you'll get a little bit of drive for a while, but there will be a lot of bad noises.
Whatever it is, it sounds like you'll have to pull the diff apart to fix it, and it will be pretty easy to see what's wrong once it's apart.