Incorrect installation of locker. I suspect incorrect seal as well, if not, then it's runout in the pinion. Who did locker install? Correct sealing after gear work is the responsibility of whoever did the work on the differential.
97 TJ Sport - 4.0/HP D30 w ARB/8.8 w LS/4.88s; 35s and flat fenders on 2.5 lift
84 CJ7; frame off restoration in progress.
The technical advice you get from me here is worth exactly what you paid me for it. Warranty issues will be reimbursed at double what you paid me for the advice.