[Beav]

The reason I asked about noise while starting is if the flexplate was broken you should/could hear approx. the same noise, just not as loud. As to if the car only made the sound in motion might have indicated a differential or possibly an axle problem.

While it's not impossible, it is abnormal for a transmission to make a clatter. Usually a clutch or band burns out, a catastrophic loss of pressure occurs, etc., none of which make much noise, if any at all.

It's terribly difficult to provide a reliable diagnosis at this distance, but it seems to be still a flexplate or transmission. Either require about the same amount of labor, but I'd rather fork over $120 for a flexplate than $2500 for a tranny (hence my prior comment.)

If you determine that the trans has to come out I'd say you probably have the ability to do the labor yourself, as you've done so well this far. Just when you get to the point of actually removing it from the car make sure you have some help, it's a lot safer that way.