Well the grinding (if they replaced posi) would be from wrong fluid. You can goto parts store and buy a tube of additive for posi's. It has special lubricants that keeps them from grinding and clicking.
Now as for needing a new one a 11k I would seriously doubt it unless you have hammered the car to death.
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My '00 GT
2000 GT Laser red-5speed,MAC Cold Air, MAC 2 1/2" Cat-Back, 3.27gears, Pro-5.0 shifter, Weld Draglite's 15x5 and 15x10, 275/50-15 DragT/A's, 165R15 front, BBK Underdrive pullies, BBK offroad X-pipe with Dynomax super converters
Best Quarter mile run to date: 13.296@101.99(9-9-01) Best MPH 102.42(9-9-01) Best 60' 1.844sec.