MAJOR driveline noise, no fwd or reverse - WTF?

[cdig]

I just picked up a 2004 GMC 2500HD, 6.0L AT. Bought it at auction for cheap (it was a theft recovery), figured i'd take a chance on it. Engine runs good, but when i put it in reverse or any drive gear, in 4wd or 2wd it doesn't even try to move, just get a very loud banging/crunching/grinding/gear stripping kind of noise... looking under the truck while it's in gear, the driveshaft isn't moving so it's not the rear diff. I'm wondering if anyone has any thoughts as to whether this would just be the tcase mainshaft broken or could it be the transmission main shaft?

My understanding is that the 4L80e shaft is pretty much bullet proof, so I'm leaning towards replacing the tcase first. Is there anyway I can diagnos this without taking the tcase out? I can try replacing the tcase first as i have a used one, but if I can diagnos it prior to doing so would be much better.

Thanks for any insight!

C.