OK good, we have the problem area. The FRONT. Now look at the diff cup on the back of the front gearbox. You'll need to remove the top plate to do this bit easier. try and roll the car on the front wheels whilst holding the brakes on, if it rolls easily then look at the cup and see if the grub screw is still there or if it's loose, if that's it then find the flat on the pinion and secure the screw again with loctite.

If not then your into the front diff for the fault, probably chewed up the internals as they dont get lubed well from the factory.