Engine Vibration 3k RPM

Hi everyone, I'm chasing an engine vibration (or at least that's what I think).

70 Dart
68 340
A500 OD transmission
Neutral converter
Neutral balancer

The vibration can be felt in Neutral when revving the engine to 3-3500k. It's not as pronounced as when it's wheels on the ground, at speed though.

The vibration goes away when the OD kicks on, which drops it to about 2200 RPM.

I thought it was driveline or pinion angle related, but I took it back to the guy who built the driveline and rear end and after we went for a drive, he doesn't believe it's driveline related. He even had another guy come by and get his opinion.

The trouble is, this is a new build (8+ years in the making) so I don't have anything to go off of.

Here are things,.I was planning on checking:
1. I was thinking maybe a misfire (not sure how to check)
2. It's got Fuel injection with timing control (maybe rotor phasing is off)

I wanted to remove driveline to be absolutely sure it's not rear end or wheel related, but not sure the best way to plug the trans. Will these work? https://www.oreillyauto.com/detail/...le-transmission-transaxle-plug-set/lis0/23400

Thanks for any suggestions.