Alright here goes, we're stumped. (me and my technician)
Coolant was leaking from the flexplate top pass side,
we replaced it, at the dealer with all tq specs etc,
Drove it 20 miles, the new flex plate broke, and the bell-housing,
Cracked all the way around, and it looks like it originated from that same passenger top side,
While I was driving it, no severe sounds were made, just two strange downshifts- not violent, however. There was a vibration around 60 mph and faster, but nothing crazy, in fact I thought it was the tires.
No sudden "crack" Didn't do anything crazy or abusive.
All mounts-transmission and engine are fine-serviceable,
The frame is not bent,
engine runs fine with no vibration,
no codes.
This is a 2003 lb7 work truck 4x4, stock, (not the same one in my signature)
165k miles, PPE programmer (set low for mileage)
What are some things we should check? What would cause such a huge failure?

I would be checking your driveshaft to see if it is out of phase....it only takes a few degrees and the more lift your truck has the quicker it will break a case. (Take it out and have it checked at a driveline shop)

Adaptation issue

Make sure both dowl are installed for the trans. What was the tq spec for the flex plate?

If coolant was leaking, you need to check the engine adaptor (Eng block to Trans).
Also check the two trans alignment dowels that Mike said.

Well first off- I was calling large alum plate that the bell housing bolts to -"intermediate plate?" that's what I was calling incorrectly the "flex" plate. It was broken in several places. Also the bell housing was cracked all the way around. Turns out the only think we can figure is that a harmonic vibration caused by a bent drive shaft was the culprit. Got the drive shaft rebalanced and we're repairing all the damage- I appreciate all the advice.
Blair~

where did you get your intermediate plate and is that the correct term for it?

Can any body tell me were the best place to get a bellhousing for my allison 1000 transmission? Mine is broken badly I can't weld it.