A833 Vibration Issue

Yup. It's all bellhousing / forward. You have? pulled the belts?

If so, next thing I'd do is yank the trans and clutch, and run it with just the flywheel. Test it thuroughly in that mode and recheck with belts off.

Take a GOOD look at engine mounts. Probably a good idea after pulling the clutch to re-install the gearbox to properly support the engine in it's "normal" mounts