727 Torqueflite stuck in third

[Bullitt-]

more of a question as I've heard & had a shudder & bang issue myself just in the driveway
... 4:10 rear probably has a posi. 
What if there's no friction modifier in the rear lube? 

[Watteee]

Hey guys!

Tested some more today. Took her for a drive, and the transmission shifts good. Vibration is still present. Starts at 1350rpm, and goes away at 1850rpm. I also feel it when I drive at 60mph (2200rpm), but when I let off the gas it stops.

The vibration at 1350rpm is also present when in neutral! Looking at the car on the lift it seems like there's no vibrating in the front, but a lot when you look at the tail shaft on the transmission.

The engine is a 400, with the cast crank. Correct balancer is mounted, and the crank / rods / pistons are weight matched, and balanced.

Could a bad converter cause a vibration at neutral as well? I changed the tranny fluid again, and I also drained the converter. No evidence of anything there.

My flex plate looks ok, and all the bolts on the converter is ok. The converter has two big weights, by the drain plug. With the converter weights pointing down, the balancer weight points up. Does this sound correct?

Any tips on what to check / do now? Change converter and pray to god?