+1 on the TrueTrac. Wiped out many of the stock Auburn units over the years. I always liked Eaton diffs (conventional clutch ones). Stronger than the Auburns. But...then I put a TrueTrac in as an experiment in one of my cars. Best thing I ever did. Going on 5+ years of winter abuse and summer fun, and not a peep of noise, locks perfectly on snow and ice. Can get a little scary on water if you aren't used to it since it's less "limited slip" and more like "limited lock".
If it's from a 9C1 car, best to assume the Auburn diff is toast. I'd just upgrade it anyways. You can smoke these stock Auburns with one uneven burnout, especially on snow, water, gravel, when the moon is full, or when the day is bright. You get my point. Throw 3.42 or 3.73 gears in it, get a new ABS reluctor, reprogram the computer and have even more fun!
These cars really wake up with 3.42 gears (or more) and a tune.