Sounds like you've tried most everything. Some Mopars came from factory with a large damper weight bolted to the rear of the transmission tailshaft to counter some hamonic vibrations in the driveline. I can't say specifically which models it was on but maybe yours needs this? Here's a picture of one style. The tails shaft housing for this damper has two 5/16" UNC bolts holes tapped diretcly underneath the output seal.