The ITVs are equipped with front and rear Eaton/Detroit Truetrac automatic limited-slip differentials. They are not switchable, nor are they "lockers". Their function is meshed helical gears that transfer greater torque to the non-spinning wheel as spin or slip increases. For more info and download links for both ITV and EATON manuals on these ITV differentials, see the thread here: ITV differentials
Note that not all ITVs have been proven to have both front and rear units installed. If you suspect yours are missing or failing (very rare) check for yours by viewing for visual identification of the differential with a bore-scope through the refill hole, or follow the EATON manual's test procedure for this specific unit.
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