Video with alignement tool
 

Did you check the prop and prop shaft for out a round or bent blades?
Does it vibrates in both gear and in N?

U-joints? Gimbal Bearing?

I highly doubt its the coupler. Prop, propshaft, u-joints or gimbal bearing are the most likely causes.

1. check the gear lube for metal.
If metal then there ya go
2. if not check the prop spines and shaft
3. pull the drive(it has to come of anyways) and check the yoke, u-joints, and coupler.
4. check the gimbal bearing
5. you should find the problem by know
Jr.

the bounce on that bar goes straight into the u-joints, would it vibrate, probably, i think you need a coupler.

We had the same problem and it was the coupler, It has a alumiumn receiver and the teeth were so bad that they were
pointed, Get the one with a steel receiver.
Mike J.

While it has never happened to me, it seems that if the engine was significantly out of alignment, this could happen as well. It's hard to imagine the coupler doing this on its own. Couplers usually get "spun" or wear out, but going out of round or somehow losing its center seems far fetched. Just my 2 cents

Rene