If it were the front of the van, it'd be a little easier to help. Especially hearing that the ball joints have been replaced. The ball joint clamp bolt can be the culprit here. If it isn't torqued down properly, the ball joint pin can move up and down in the clamp and make such a knocking sound. Also, if you have a loose or broken motor mount you'll here such sounds.
If it's really com> I can't seem to determine the source of a noise in this van. The noise
occurs whenever the steering is changed abruptly, either left or
> right. It is a deep "KA-CLUNK" sound. It seems to be coming from the
> rear of the van when turning. Sometimes it will be a single "CLUNK"
> but many times it is a double "KA-CLUNK" sound. Sometimes I will hear
> a single "CLUNK" when the rear, especially the left rear tire, strikes
> a bump in the road.
>
> At first I thought it might be the spare tire swinging from it's chain
> beneath the van. I could move it with my foot, so I tightened it
> further. No improvement.
>
> Then, I thought it might be loose wheel bearings in the rear. There
> was a small amount of play in each but I tightened them further, still
> leaving a minor amount of free play. No improvement.
>
> The exhaust system can be shaken to the extremes, back and forth
> without contacting other components.
>
> The shocks have been replaced, the ball joints have been replaced, the
> struts have been replaced.
>
> The leaf springs and axle appear secure at all points.
>
> What could be causing this "KA-CLUNK" on quick turning? -- Thanks.