It didn't fall out completely, it just detached from the differential and dragged on smooth asphalt for a couple hundred feet. There's very little damage, I only lost two caps on the ujoint and chewed up the straps... I have straps on the way and already have a new ujoint...

After I rebuilt the transmission and installed the driveshaft I wasn't really sure if it was on the right way with the weight that's on it... I never really noticed a vibration after the install but because I forgot to put the clips on the caps the ujoint came apart from the differential I didn't really have time to figure out if the balance was off with the driveshaft...

I'll try putting the rear on jacks, I have 6 ton jacks and I have lot's of property incase it runs from me!
I haven't balanced the tires but they are used tires on brand new rims...
Thanks Gene, I'll perform the test you outlined.