1968 Dart sudden rear end noise.

Wow. I didn't think these rear ends were absolute trash that coukdnt be rebuilt when they fail. Chrysler put tons of these in A bodies and I've seen people state they lasted a couple hundred thousand miles. If they were such junk why did Chrysler put these behind 273's and 318's knowing they wouldn't hold up? B bodies never used this rear end I take it. My next question is if it's all doom and gloom and the rear end can't be properly rebuilt to last,how hard to find is an 8 3/4" rear end that will drop on with no modification?.
Smallest rear axle in a b body was 8.25 to the best of my knowledge. The 7.25 was a very light duty piece that would last forever behind a slant 6, but it doesn't live as long with a V8, even a bone stock 273 or 318 has enough torque to wear it out. Lots of a bodies had 8.25 differentials, but I am pretty sure they were all big bolt pattern, which of course presents a new problem in that your 68 is likely small bolt pattern axles.