2004 Cadillac CTS Rear Diff output shaft seal leak

I have a 2004 Cadillac CTS 3.6 L with a Getrag open differential. At 29,000 miles, the driver's side output shaft seal has sprung a small leak (dime sized puddle on garage floor after driving). I was quoted $355 to replace the seal (labor/part). My question: I read that adding 1/3 bottle of stop engine leak to the diff may swell the seal and stop the leak after 500 - 1,000 miles. Is this a viable option to try first, or should I go with replacing the seal?