So I was accelerating up an on ramp, going from 3rd to 4th gear, when I hear a big BOOM come from under the car. Immediately following the boom, there was a horrible noise coming from under the car, like a wurring noise, metal spinning on metal.. torn up transmission noises.. I took the car to the local Mazda garage, and they said my diff went bad. my question is, should i replace it with a used one? new OEM, or new aftermarket?

thanks in advance..

 

Replace it with whatever you can afford. Used OEM will be the cheapest and easiest to find. Check Mazmart.com for pricing, or one of the part out threads.