FWIW from personal experience:
A light clunking - more like a rotational rattle - was a u-joint breaking up.
Heavy clunking - again rotational - was a ring & pinion, specifically teeth broken off on the ring gear.
In both cases, made it home, but the ring gear event sounded bad, I could feel it as much as hear it.
Oh, there was another, again rotational, when a shop under-torqued the pinion nut on my Dana 60; that was more of a lighter clunking, all the time.
That one I caught quickly, with no resulting damage.