I don't know much about this so someone else chime in. I believe with the trac-loc there is a clutch inside of the diff that allows for an open diff when cornering and limited slip when there isn't much tracking. After x number of miles the clutch wears out and leaves you with the equivalent of an open diff. From what I can tell it is as easy as replacing that part.
Mileage depends on a lot of things. The track lock is indeed a clutch style posi and will eventually wear out as will all clutch style posi diffs. You can try changing the gear lube and NOT adding any friction modifyer. This will tend to make the clutches grip more. Give it a week or two to work into the clutch material and replace the old lube that was soaked in. If this does not help you can buy a clutch kit and rebuild the trac-loc.
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