2014 rear end

Pretty much bought a truck without driving it, I say pretty m7ch because I havnt signed any papers yet.
It makes a high pitch whining noise when light on the throttle or at a constant speed 70 -100km. When I'm off the throttle it's quiet as the tires let it be.
Took it to a Toyota dealer to check out and as I thought the ring and pinion are deemed the problem.
They are saying it's cheaper to replace the whole assembly then just the ring and pinion, is this the way it is?
Truck has high km's 253k(150ish miles) 5.7 4x4. Other than that I'm liking the drive, a lot bigger truck then my 05 tundra.
If I could handle the noise I would keep driving it, but thinking it could damage more then what's going on now.
In Canada it's approx. 2600 for new assembly and labor.