This now sounds even more like I thought in the last reply, your front diff internals have either let go or the diff is too far off the pinion gear, pull the diff and open it up to examine it, if not the diff then you need to shim it onto the pinion gear a little more.

Or you've lost one of the front driveshafts.


NOTE: When you pull the diff out, before opening it, hold the body and one of the cups still and try to spin the other, it shouldn't move, then try it with the other cup. If either of them spin then it's definately the diff. Either a sun pin has sheared or the teeth have gone south on the suns or planets.