Yes the Trac-loc wears out, however it just stops working and acts like open when this happens. You have something wrong with the carrier, it could be the clutch packs, could also be the spider gears or the side gears.
Did they pull the cover and inspect it when they changed the fluid or just pump out the old and put in new? If they did not pull the cover then that would be the first step - you may have a broken piece of clutch in there causing more damage as time goes by.
Friends don't let Friends run Bosch