I agree: Check your clutch / liner / clutch bell first.
The liner does wear out
You may have noticed while flying that a full collective climbout resulted in what may hove sounded like the motor over-reving.
If you run a throttle curve, that was the clutch slipping because the liner is worn out.
If you run a gov, then the motor will not over rev, but you may see that the blades are visibly slowing down while under a heavy cyclic or collective load.
But, I am not sure it is related to the tail vibration.
I would start with the tail rotor hub and work your way forward on the heli.
It is easy to tap the tail blade on the grass and that will tweak the tail rotor hub - they are cheap to replace.
Also, is this is a torque tube, check the TT bearing inside the boom. They wear out too.
Then, the usual suspects: The autorotation gear, the front tail rotor drive gear, the umbrella gears (if TT).
As for the hula dance at takeoff - that could be loose blades (the blade bolt isn't tight enough), the head dampers are worn (allowing the feathering shaft to shift) or a bent main mast.
It could also be the FBL gyro reacting to the vibration.
Any vibration on the airframe is transmitted to the gyro.
And, if the gyro's gain is set too high, and there is a new vibration, it starts to react in a way you haven't seen before.
One area I always look at is the servo wire bundle that leads to the gyro.
If you did a neat and tidy job of routing the wires into a neat a tidy bundle as it connects to the gyro, you have set yourself up for a vibration induced oscillation in the gyro.
The bundle starts to vibrate and transmit that vibration into the gyros case.
I learned that the hard way.
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