I am thinking about getting a set of good used gears 3.31 for my BS rear (aftermarket 4.10s now).

Can used gears be set up correctly again?

I have richmonds now in my other car and they whine a bit (surprising :)). US Gear 3.42s are my next choice.

There is nothing wrong with used gears as long as they are in good shape and set up correctly.

It is more difficult to set up used gears because the contact pattern is harder to read. In a case like the 3.31 gears where they are not available new any more, used gears are your only choice. If you are careful and set them up correctly then they should be fine. Pay more attention to the coast pattern than the drive pattern because it will give you a more accurate reading on used gears. The coast side gets much less wear than the drive side.

If you are removing the gears from another housing check the pinion depth and backlash first. Then duplicate the measurements in your housing.

I am getting the gears via EBAY :) and replacing the 4.10s and 4 series posi with the 3.31 gears and another used 3 series posi.