What's broke?

AJ/FormS said

If the engine is the source of the vibration, it often happens that said vibration causes the cluster to rattle amongst the gears and specifically against the Input gear.
Therefore, I would look to the engine FIRST before dropping the trans.
The most common thing to cause a vibration of the type you describe, would be a contaminant in the fuel.
The second most common thing would be debris in the idle bleeds.

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Understood and agreed. That's what bothers me most about the matter...that it's maybe engine based not transmission based. I have spent time looking for the problem from an engine perspective, mostly spark; wires, rotor, cap, boots and such. I have not found anything wrong to date...but I am certainly aware of the possibility.

Air bleeds never occurred to me. I'll take a look at them, give 'em a flush.

Thanks.