Don't know what you have changed recently ( if anything - besides the u-joints - which didn't solve the problem ) to get the vibration, but you could try unbolting the convertor and rotating it 180 degrees and bolting it back up. The flex plate does have a big bite out of it for external balanced engines right? Got a big hunk of iron hanging off the trans tailshaft? That was Chryslers cure for a vibration in about that rpm range on some vehicles.