Allison Transmission

I have an Allison MT643 transmission in a 1999 Winnebago Motorhome.

On the very rear of this transmission there is a fly wheel about an inch thick and about 12 inches in diameter. The drive shaft universal joint is attached to this flywheel

Oil is leaking out on the universal joint side of this flywheel. The transmission side is dry.

What is causing this leak ?