90% of the time, a broken flex plate is caused by loose flywheel housing or converter housing bolts. If you've already pulled the trans, then you would have noticed whether or not the converter housing bolts were loose. Also while you're at it, check and make sure that the bolts holding the flywheel housing to the engine are tight. Look closely at the motor mounts too.
And check the vibration damper- these are a simple design, and it will be obvious if the thing is bad. Check and see if the rubber is swelled (from oil) or cracked... make sure it isn't wobbling. With 10,000 hours- it might be a good idea to replace the thing anyway.
But what has me puzzled is that you say the loader "had real low power and overheating the trans filter". I would think that a broken flex plate would yield NO power rather than just low power... and I don't know what to make of "overheating the trans filter". I have to wonder if there's some poor communication here, or if there are more underlying problems.