Powerglide Transmission Leak

[brad4176455]

I have been having some trouble with my tansmission leaking latley. So today i got under the truck and tried to find the problem. I noticed a fresh drip of tranny fluid coming off of the speedometer cable were it goes into the transmission. I then notice that the nut on the fittling going into the transmission was loose. so i tried tightening it. But once i did that i noticed that the fitting that the nut threaded onto was loose. I could wiggle it with my hand. I was just wanting to know if there is a seal or and o-ring or something in there that would be causing it to leak. Also is it easy to replace? I am not very familiar with transmission problems.