A used, unknown diff is a crap shoot. If you trust the seller and he/she actually had it in use and can say it was good, that's best, or in a car you can test before buying.
Rebuilding diffs is time-consuming and expensive, especially if you want the LS feature shimmed to work, which it won't be as removed. I'd figure on at least $500 in CA, and it still can be a crap shoot on how quiet the gears will be in mesh.
APE is great, but the same caveat applies; they don't always know how each diff was used. In general, a late-model low mileage diff can be expected to run well, but you never know. And all the late ones (Bosch Spiders) are 4.10. APE in my experience stands behind what they sell.
Barber Shop has a very mixed reputation. Alex there is good. I wouldn't go there myself.
Andrew