Axle movement?

Okay so the other day I pushed my 68 dart out of my garage and into my driveway. Why I didn't drive it? It was 3am with only exhaust manifolds on hahaha Anyways, I noticed a random vibration in the rear by the tires. I thought just maybe I put the wheels on a little crooked, so I thought that maybe they were moving around. So I jacked the car up and i tried to jiggle the rear tire to look for play from maybe the tire being on crooked. I got none. I then pulled towards me and back towards the car, and I got movement. Took the wheels off and the drum cover. I then would move the (axle?) in and out a little.

Sorry I don't know exactly the terminology for the rear.

I went ahead and recorded 2 videos.
[ame]http://youtu.be/SM0SeIN57V4[/ame]

I will say that the last time I drove the car, which was about 2 weeks ago, it drove fine and I didn't feel any vibration by the rear wheels. I don't know if this is normal or not.

Car is a 68 dart with a 73 duster 8 1/4 rear end.