P.S. - This happened almost exactly where @Monty5150 picked up a rod knock a few weeks ago. 


Limiting tech in the lounge. The crank is scrap, the block itself needs to be checked if it can be saved or not.
Getting the tech out of the lounge...
I know the crank is scrap, but I'm not sure the block is worth saving. I think it's easier to find a complete engine than a crankshaft. But I haven't started looking yet. I'm not sure what direction I want to go.
Parts are available, if you're willing to look and wait some. Finding a decent running takeout is probably your best bet as far as cost and down time. I recall seeing running engines between 500 and 1000 bucks. That's going to be your earliest route to go.
Repower with different engines are easy on these trucks as you know. Mainly comes down to cost and time.

Broken crank is the only explanation?
Have you tried turning it off and back on again?
My older brother had an Escort GT he raced on ice covered ponds and lakes in NH ,and it broke the crank between 2 cylinders,after running it wide open with no rev limiter at least 2 seasons--it still ran and drove,just had a horrible clatter and though he finished a race that way with it,he feared it would lock up suddenly..
He pulled the engine apart later on at home and saw the crank did not just shear off cleanly,it was like a finger joint in wood,and the pieces would still lock together well enough to let the whole crank still turn as a unit--with some free play,much like yours has in it..
I'm pretty sure the crank in the 6.2 I got from my friends ramp truck snapped in the same manner..
What locates the crank? Can it be explained by a loose balancer? Don't get me wrong, it does sound like something bad has let go, but anything that lets the crank move fore/aft can let stuff hit and clatter.
Can't you easily drop the pan?
Can it be explained by a loose balancer?
If I hafta swap, the new engine oughta be better than the old one.
I have a local buddy selling a late-model replacement 6.5 crate engine with 40k miles, mounted in a '94 truck with no transmission. Perfect donor if I want to stick with the 6.2 family. If.