two items.
1--I have a leak out the tailshaft seal of the AOD. I assume it is as easy as getting a new seal and installing.

2--Looking at the front yoke of the driveshaft, there is a large round metal piece, that appears to have a rubber spacer installed, and a second piece around that. Kind of like a harmonic balancer with the two pieces of metal bonded together with a rubber ring. It's part of the front u-joint yoke.
What is that thing for? Just NVH?

 

I think it's a warning parts, ex: if you're universal joint become noisy it will trandfert the sound to that part so you can easely hear it before it break in parts and be dangerous

 

That part is a harmonic balancer. It is not a warning part.

The leak may not be just as easy as replacing the seal. It could be the seal, but the seal may have failed due to a worn bushing in the extension housing. You need to remove the driveshaft, then the housing, and inspect the bushing and seal. Replace what's worn.

 

When you remove the driveshaft examine the slip yoke surface that rides in the seal/bushing area. Any roughness on the yoke will give you a clue as to the condition of the bushing. As Casey said above the seal doesn't usually fail unless there is a problem with the yoke/bushing. If you need to replace the bushing don't be supprised if you also need to replace the yoke.

 

Thanks all.