With a limited slip both wheels should turn the same direction when jacked up. The clutch pack in the diff. acts as a lock but allows the wheels to slip slightly (limited) to make it easier when going around corners. I would say that if one wheel is spinning under power that the clutch pack is going out. I have never been into a MB diff. but common sense tells me that the power coming into the diff is not being distriputed equally to both wheels. The way it get distributed is through the clutch disks. If they slip power only goes to one side.
ps. mbtjc- Remember, not all wine gets better with age-some turns into vinegar.