Noisy rearend 69 barracuda 318 car

Other than a date code, there are no "numbers" on a rear end they are not serialized to the Vin as engines and transmissions are. The problem of the replacement having the same issue is possible, more so if buying from stranger on Craigslist. But dealing with fellow FABO members should eliminate that. Things to check so you don't end up with the same problem on a used rear are:

1) the pinion assembly should not move in and out at all (towards/away from ring gear)
2) pinion assembly should turn freely with no binding/tight spots thru one full revolution of the ring gear ( i.e. you'd rotate the pinion 3.55 revolutions for one full ring gear revolution on a rear with a 3:55 gearset)
3) the amount of free play when turn rotating the driveshaft yoke back and forth (without rotating the ring gear) should be in the thousandths not fractions

But nothing is guaranteed, and my quick checks are far from 100% guaranteed. No right or wrong, you just have to prioritize the various factors such as cost, originality, etc. are most important to you and proceed.