Land Rover 110 PU Drivetrain Vibration

Hello Experts,

I have just had the transfer box of my 110 pick up replaced. The vehicle now had a vibration between 60 and 75 km/h (37 -47 mph).
I have checked the balance of the tire balance - all good;
The propshafts have been balanced - all good;
The phasing of the propshafts is good, all arrows aligned;

If I remove the rear Propshaft, the vibration disappears.

What could be causing this?

Check the following
Handbrake drum. Is this tight? When the prop goes on is it still tight?
The UJs in the rear prop may have worn & need replacing.
Is the big nut on the transfer box(behind the propshaft) tight?