I've had a few flex plates and converters break on my Yales. Symptoms just as you described. Correct repair requires separating engine and transmission to replace the flex plate. Replace the converter seal while you're in there, 'cause if it's not leaking already, it will be soon.

I've also had torque converter lugs break off. Again, separate engine and trans to R&R. Converters can be repaired and re-balanced. Your local transmission shops probably have a resource for this. Much cheaper than a new converter.

On some older trucks that I'm just nursing along, I've had my maintenance guy reach in there with a welder and tack it in place. Generally holds up just fine. Stops the rattle anyway. Not a real elegant repair though.